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/update/nxtitemp.c

@@ -47,16 +47,16 @@ nxtitemp(struct nstr_item *np)
     int selected;
 
     if (np->sel == NS_UNDEF)
-	return 0;
+	return NULL;
     do {
 	if (np->sel == NS_LIST) {
 	    np->index++;
 	    if (np->index >= np->size)
-		return 0;
+		return NULL;
 	    np->cur = np->list[np->index];
 	} else if (np->sel == NS_CARGO) {
 	    if (np->next < 0)
-		return 0;
+		return NULL;
 	    np->cur = np->next;
 	    np->next = unit_cargo_next(np->type, np->next);
 	} else {
@@ -64,7 +64,7 @@ nxtitemp(struct nstr_item *np)
 	}
 	gp = ef_ptr(np->type, np->cur);
 	if (!gp)
-	    return 0;
+	    return NULL;
 
 	selected = 1;
 	switch (np->sel) {
@@ -74,7 +74,7 @@ nxtitemp(struct nstr_item *np)
 	    break;
 	case NS_DIST:
 	    if (CANT_HAPPEN(!(ef_flags(np->type) & EFF_XY)))
-		return 0;
+		return NULL;
 	    if (!xyinrange(gp->x, gp->y, &np->range)) {
 		selected = 0;
 		break;
@@ -85,25 +85,25 @@ nxtitemp(struct nstr_item *np)
 	    break;
 	case NS_AREA:
 	    if (CANT_HAPPEN(!(ef_flags(np->type) & EFF_XY)))
-		return 0;
+		return NULL;
 	    if (!xyinrange(gp->x, gp->y, &np->range))
 		selected = 0;
 	    break;
 	case NS_XY:
 	    if (CANT_HAPPEN(!(ef_flags(np->type) & EFF_XY)))
-		return 0;
+		return NULL;
 	    if (xnorm(gp->x) != np->cx || ynorm(gp->y) != np->cy)
 		selected = 0;
 	    break;
 	case NS_GROUP:
 	    if (CANT_HAPPEN(!(ef_flags(np->type) & EFF_GROUP)))
-		return 0;
+		return NULL;
 	    if (np->group != gp->group)
 		selected = 0;
 	    break;
 	default:
 	    CANT_REACH();
-	    return 0;
+	    return NULL;
 	}
 	if (selected && np->ncond) {
 	    /* nstr_exec is expensive, so we do it last */

src/lib/update/plague.c

@@ -99,7 +99,7 @@ do_plague(struct sctstr *sp, struct natstr *np, int etu)
 	}
     }
     if (sp->sct_item[I_CIVIL] == 0 && sp->sct_item[I_MILIT] == 0
-	&& !has_units(sp->sct_x, sp->sct_y, sp->sct_own, 0)) {
+	&& !has_units(sp->sct_x, sp->sct_y, sp->sct_own, NULL)) {
 	makelost(EF_SECTOR, sp->sct_own, 0, sp->sct_x, sp->sct_y);
 	sp->sct_own = 0;
 	sp->sct_oldown = 0;

src/lib/update/populace.c

@@ -52,7 +52,7 @@ populace(struct natstr *np, struct sctstr *sp, int etu)
 	sp->sct_oldown = sp->sct_own;
     }
     if (!civ && !mil && !sp->sct_item[I_UW]
-	&& !has_units(sp->sct_x, sp->sct_y, sp->sct_own, 0)) {
+	&& !has_units(sp->sct_x, sp->sct_y, sp->sct_own, NULL)) {
 	makelost(EF_SECTOR, sp->sct_own, 0, sp->sct_x, sp->sct_y);
 	sp->sct_own = 0;
 	sp->sct_oldown = 0;

src/lib/update/revolt.c

@@ -352,7 +352,7 @@ guerrilla(struct sctstr *sp)
     }
   domove:
     if (move && che > 0) {
-	struct sctstr *nicest_sp = 0;
+	struct sctstr *nicest_sp = NULL;
 	if (convert)
 	    min_mil = 999;
 	else

src/lib/update/sail.c

@@ -299,7 +299,7 @@ void
 sail_ship(natid cn)
 {
     struct shpstr *sp;
-    struct fltheadstr *head = 0;
+    struct fltheadstr *head = NULL;
     struct fltheadstr *fltp;
     int n;
 

src/lib/update/sect.c

@@ -297,7 +297,7 @@ produce_sect(int natnum, int etu, struct bp *bp, long p_sect[][2])
 	 * they all starved or were plagued off.
 	 */
 	if (vec[I_CIVIL] == 0 && vec[I_MILIT] == 0 &&
-	    !has_units(sp->sct_x, sp->sct_y, sp->sct_own, 0)) {
+	    !has_units(sp->sct_x, sp->sct_y, sp->sct_own, NULL)) {
 	    if (!player->simulation) {
 		makelost(EF_SECTOR, sp->sct_own, 0, sp->sct_x, sp->sct_y);
 		sp->sct_own = 0;