Don't use 0 as null pointer constant, part 1

Use NULL instead of 0, for clarity.  Except in pointer comparisons;
leave that to the next two commits.

src/lib/commands/assa.c

@@ -74,7 +74,7 @@ assa(void)
 	return RET_SYN;
     if (!sarg_xy(p, &def->x, &def->y))
 	return RET_SYN;
-    if (att_abort(A_ASSAULT, 0, def))
+    if (att_abort(A_ASSAULT, NULL, def))
 	return RET_FAIL;
 
     /*
@@ -110,7 +110,7 @@ assa(void)
     if (att_abort(A_ASSAULT, off, def)) {
 	pr("Assault aborted\n");
 	att_empty_attack(A_ASSAULT, 0, def);
-	return att_free_lists(&olist, 0);
+	return att_free_lists(&olist, NULL);
     }
 
     /* If we're assaulting our own sector, end here */
@@ -127,7 +127,7 @@ assa(void)
     if (att_abort(A_ASSAULT, off, def)) {
 	pr("Assault aborted\n");
 	att_empty_attack(A_ASSAULT, 0, def);
-	return att_free_lists(&olist, 0);
+	return att_free_lists(&olist, NULL);
     }
 
     /*

src/lib/commands/atta.c

@@ -73,7 +73,7 @@ atta(void)
 	return RET_SYN;
     if (!sarg_xy(p, &def->x, &def->y))
 	return RET_SYN;
-    if (att_abort(A_ATTACK, 0, def))
+    if (att_abort(A_ATTACK, NULL, def))
 	return RET_FAIL;
 
     /* Show what we're attacking, and check treaties */
@@ -84,7 +84,7 @@ atta(void)
     /* Ask about offensive support */
 
     att_ask_support(2, &fort_sup, &ship_sup, &land_sup, &plane_sup);
-    if (att_abort(A_ATTACK, 0, def)) {
+    if (att_abort(A_ATTACK, NULL, def)) {
 	att_empty_attack(A_ATTACK, 0, def);
 	return RET_OK;
     }
@@ -110,14 +110,14 @@ atta(void)
     if (att_abort(A_ATTACK, off, def)) {
 	pr("Attack aborted\n");
 	att_empty_attack(A_ATTACK, 0, def);
-	return att_free_lists(&olist, 0);
+	return att_free_lists(&olist, NULL);
     }
 
     ototal = att_get_offense(A_ATTACK, off, &olist, def);
     if (att_abort(A_ATTACK, off, def)) {
 	pr("Attack aborted\n");
 	att_empty_attack(A_ATTACK, 0, def);
-	return att_free_lists(&olist, 0);
+	return att_free_lists(&olist, NULL);
     }
 
     /*

src/lib/commands/boar.c

@@ -136,14 +136,14 @@ boar(void)
     if (att_abort(A_BOARD, off, def)) {
 	pr("Board aborted\n");
 	att_empty_attack(A_BOARD, 0, def);
-	return att_free_lists(&olist, 0);
+	return att_free_lists(&olist, NULL);
     }
 
     ototal = att_get_offense(A_BOARD, off, &olist, def);
     if (att_abort(A_BOARD, off, def)) {
 	pr("Board aborted\n");
 	att_empty_attack(A_BOARD, 0, def);
-	return att_free_lists(&olist, 0);
+	return att_free_lists(&olist, NULL);
     }
 
     /*

src/lib/commands/bomb.c

@@ -122,7 +122,7 @@ bomb(void)
     (void)pathtoxy(flightpath, &tx, &ty, fcost);
     pr("target sector is %s\n", xyas(tx, ty, player->cnum));
     getsect(tx, ty, &target);
-    ip = 0;
+    ip = NULL;
     ap_to_target = strlen(flightpath);
     if (flightpath[ap_to_target - 1] == 'h')
 	ap_to_target--;

src/lib/commands/buil.c

@@ -540,7 +540,7 @@ build_bridge(struct sctstr *sp, short *vec)
 
     if ((val = chkdir(*p, DIR_FIRST, DIR_LAST)) < 0) {
 	pr("'%c' is not a valid direction...\n", *p);
-	direrr(0, 0, 0);
+	direrr(NULL, NULL, NULL);
 	return 0;
     }
     newx = sp->sct_x + diroff[val][0];
@@ -816,7 +816,7 @@ build_tower(struct sctstr *sp, short *vec)
 
     if ((val = chkdir(*p, DIR_FIRST, DIR_LAST)) < 0) {
 	pr("'%c' is not a valid direction...\n", *p);
-	direrr(0, 0, 0);
+	direrr(NULL, NULL, NULL);
 	return 0;
     }
     newx = sp->sct_x + diroff[val][0];

src/lib/commands/coas.c

@@ -70,7 +70,7 @@ coas(void)
     if (!snxtsct(&nstr, player->argp[1]))
 	return RET_SYN;
     for (i = 0; i < TSIZE; i++)
-	list[i] = 0;
+	list[i] = NULL;
     cp = malloc(sizeof(*cp));
     snxtitem_all(&ni, EF_SHIP);
     while (nxtitem(&ni, &cp->c_shp)) {
@@ -139,18 +139,18 @@ static int
 showship(struct coast **cpp, int x, int y)
 {
     struct coast *cp;
-    struct coast *todelete = 0;
+    struct coast *todelete = NULL;
     struct coast **prev;
     int nship = 0;
 
-    prev = 0;
+    prev = NULL;
     cp = *cpp;
     prev = cpp;
     do {
 	/* we delete it, we free it. */
 	if (todelete) {
 	    free(todelete);
-	    todelete = 0;
+	    todelete = NULL;
 	}
 	if (cp->c_shp.shp_x != x || cp->c_shp.shp_y != y) {
 	    prev = &(*prev)->c_next;

src/lib/commands/expl.c

@@ -119,7 +119,7 @@ explore(void)
 	pr("Only exploring with %d.\n", amount);
     }
 
-    if (!want_to_abandon(&sect, vtype, amount, 0)) {
+    if (!want_to_abandon(&sect, vtype, amount, NULL)) {
 	pr("Explore cancelled.\n");
 	return RET_FAIL;
     }

src/lib/commands/flash.c

@@ -124,7 +124,7 @@ sendmessage(struct natstr *us, struct natstr *to, char *message, int verbose)
 
     time(&now);
     tm = localtime(&now);
-    for (other = player_next(0); other != 0; other = player_next(other)) {
+    for (other = player_next(NULL); other; other = player_next(other)) {
 	if (other->state != PS_PLAYING)
 	    continue;
 	if (to && other->cnum != to->nat_cnum)

src/lib/commands/lboard.c

@@ -107,14 +107,14 @@ lboa(void)
     if (att_abort(A_LBOARD, off, def)) {
 	pr("Land unit boarding aborted\n");
 	att_empty_attack(A_LBOARD, 0, def);
-	return att_free_lists(&olist, 0);
+	return att_free_lists(&olist, NULL);
     }
 
     ototal = att_get_offense(A_LBOARD, off, &olist, def);
     if (att_abort(A_LBOARD, off, def)) {
 	pr("Land unit boarding aborted\n");
 	att_empty_attack(A_LBOARD, 0, def);
-	return att_free_lists(&olist, 0);
+	return att_free_lists(&olist, NULL);
     }
 
     /*

src/lib/commands/load.c

@@ -706,7 +706,7 @@ load_comm_ship(struct sctstr *sectp, struct shpstr *sp,
 			   load_unload, atoi(p));
     if (!load_comm_ok(sectp, sp->shp_own, item, move_amt))
 	return RET_OK;
-    if (!want_to_abandon(sectp, item, move_amt, 0))
+    if (!want_to_abandon(sectp, item, move_amt, NULL))
 	return RET_FAIL;
     if (!still_ok_ship(sectp, sp))
 	return RET_SYN;

src/lib/commands/move.c

@@ -140,7 +140,7 @@ move(void)
 	}
     }
 
-    if (!istest && !want_to_abandon(&sect, vtype, amount, 0)) {
+    if (!istest && !want_to_abandon(&sect, vtype, amount, NULL)) {
 	pr("Move cancelled.\n");
 	return RET_FAIL;
     }

src/lib/commands/para.c

@@ -128,7 +128,7 @@ paradrop(struct emp_qelem *list, coord x, coord y)
     att_combat_init(def, EF_SECTOR);
     def->x = x;
     def->y = y;
-    if (att_abort(A_PARA, 0, def))
+    if (att_abort(A_PARA, NULL, def))
 	return RET_FAIL;
 
     /* Show what we're air-assaulting, and check treaties */

src/lib/commands/play.c

@@ -47,7 +47,7 @@ play(void)
     int saw = 0;
 
     play_header();
-    for (joe = player_prev(0); joe; joe = player_prev(joe)) {
+    for (joe = player_prev(NULL); joe; joe = player_prev(joe)) {
 	saw += play_list(joe);
     }
     if (player->god || opt_BLITZ)

src/lib/commands/reco.c

@@ -97,12 +97,12 @@ reco(void)
     /*
      * now arm and equip the bombers, transports, whatever.
      */
-    pln_arm(&bomb_list, ap_to_target, 'r', 0, P_S | P_I);
+    pln_arm(&bomb_list, ap_to_target, 'r', NULL, P_S | P_I);
     if (QEMPTY(&bomb_list)) {
 	pr("No planes could be equipped for the mission.\n");
 	return RET_FAIL;
     }
-    pln_arm(&esc_list, ap_to_target, 'r', 0, P_F | P_ESC);
+    pln_arm(&esc_list, ap_to_target, 'r', NULL, P_F | P_ESC);
     ac_encounter(&bomb_list, &esc_list, ax, ay, flightpath,
 		 *player->argp[0] == 's' ? PM_R | PM_S : PM_R);
     if (QEMPTY(&bomb_list)) {

src/lib/commands/skyw.c

@@ -68,7 +68,7 @@ skyw(void)
     if (!snxtsct(&nstr, player->argp[1]))
 	return RET_SYN;
     for (i = 0; i < TSIZE; i++)
-	list[i] = 0;
+	list[i] = NULL;
     skyp = malloc(sizeof(*skyp));
     snxtitem_all(&ni, EF_PLANE);
     while (nxtitem(&ni, &skyp->s_sat)) {
@@ -131,20 +131,20 @@ static int
 showsat(struct sky **skypp, int x, int y)
 {
     struct sky *skyp;
-    struct sky *todelete = 0;
+    struct sky *todelete = NULL;
     struct sky **prev;
     struct plchrstr *pcp;
     char *name;
     int nsat = 0;
 
-    prev = 0;
+    prev = NULL;
     skyp = *skypp;
     prev = skypp;
     do {
 	/* we delete it, we free it. */
 	if (todelete) {
 	    free(todelete);
-	    todelete = 0;
+	    todelete = NULL;
 	}
 	if (skyp->s_sat.pln_x != x || skyp->s_sat.pln_y != y) {
 	    prev = &(*prev)->s_next;

src/lib/commands/stre.c

@@ -82,7 +82,7 @@ stre(void)
 	    pr("%7s", "");
 
 	if (def->sct_type != SCT_MOUNT)
-	    r_total = att_reacting_units(def, 0, 0, &dummy, 9999999);
+	    r_total = att_reacting_units(def, NULL, 0, &dummy, 9999999);
 	else
 	    r_total = 0.0;
 	def->own = 0;