include the PM_ index in mons (permonst)

This is useful for debugging and it allows the index
to be used directly instead of calculated in a
monsndx() function, which has been removed.

I left monsndx() in as a simple short-hand macro for the value
and didn't change the use cases, the reasoning being that this:
    monsndx(mon->data)
is arguably a little easier on the eyes than:
    mon->data->pmidx

LOW_PM, NON_PM, SPECIAL_PM have been included in the 'enum monnums'
now, instead of as individual macro definitions.

I chose to add the pmidx field as an instance of the enum declaration,
because that has very advantageous results in some debuggers, where it is
then shown as:
    pmidx PM_GRAND_MASTER (349) monnums
instead of the less-informative:
    pmidx 349 int

Adding the element count to the extern declaration for mons from:
    'extern struct permonst *mons[];'
to the more specific declaration to that in src/monst.c:
    'extern struct permonst *mons[NUMMONS + 1];'
then allows navigation through the mons array in one of the debuggers.

src/mondata.c

@@ -771,26 +771,6 @@ same_race(struct permonst *pm1, struct permonst *pm2)
     return FALSE;
 }
 
-DISABLE_WARNING_UNREACHABLE_CODE
-
-/* return an index into the mons array */
-int
-monsndx(struct permonst *ptr)
-{
-    register int i;
-
-    i = (int) (ptr - &mons[0]);
-    if (i < LOW_PM || i >= NUMMONS) {
-        panic("monsndx - could not index monster (%s)",
-              fmt_ptr((genericptr_t) ptr));
-        /*NOTREACHED*/
-        return NON_PM; /* will not get here */
-    }
-    return i;
-}
-
-RESTORE_WARNING_UNREACHABLE_CODE
-
 /* for handling alternate spellings */
 struct alt_spl {
     const char *name;