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

(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.