(pln_maxno): Variable's value is well-known constant N_MAXPLANE + 1.

Useless, remove.  Users changed to check plchr's sentinel, because
that's clearer than comparing the index to N_MAXPLANE or sizeof(plchr)
/ sizeof(*plchr).

(plchr): Change sentinel's pl_name to NULL.

include/plane.h

@@ -144,7 +144,6 @@ struct plist {
 	(struct plnstr *) ef_ptr(EF_PLANE, n)
 
 extern struct plchrstr plchr[N_MAXPLANE + 1];
-extern int pln_maxno;
 
 struct shiplist {
     short uid;

src/lib/global/init.c

@@ -76,7 +76,7 @@ init_plchr(void)
     struct plchrstr *pp;
     int pcount;
 
-    for (pp = plchr, pcount = 0; pcount < pln_maxno; pcount++, pp++) {
+    for (pp = plchr; pp->pl_name; pp++) {
 	if (pp->pl_flags & P_M)
 	    pp->pl_flags |= P_V;
     }

src/lib/global/plane.c

@@ -301,9 +301,8 @@ struct plchrstr plchr[] = {
      0,
      P_N | P_M},
     {
-     "", 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+     NULL,
+     0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
      0,
      0}
 };
-
-int pln_maxno = (sizeof(plchr) / sizeof(struct plchrstr)) - 1;

src/lib/subs/show.c

@@ -97,7 +97,7 @@ make_new_list(int tlev, int type)
 
     lookup_list_cnt = 0;
     if (type == EF_PLANE) {
-	for (pp = plchr, count = 0; count < pln_maxno; count++, pp++) {
+	for (pp = plchr; pp->pl_name; pp++) {
 	    if (pp->pl_tech > tlev)
 		continue;
 	    if (pp->pl_name == 0 || pp->pl_name[0] == '\0')

src/server/main.c

@@ -495,7 +495,7 @@ nullify_objects(void)
 	dchr[SCT_OIL].d_cost = -1;
 	dchr[SCT_REFINE].d_cost = -1;
     }
-    for (i = 0; i < pln_maxno; i++) {
+    for (i = 0; i < plchr[i].pl_name; i++) {
 	if (opt_NO_HCMS)
 	    plchr[i].pl_hcm = 0;
 	if (opt_NO_LCMS)