(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;