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/allmain.c

@@ -1110,6 +1110,9 @@ timet_delta(time_t etim, time_t stim) /* end and start times */
 struct enum_dump monsdump[] = {
 #include "monsters.h"
     { NUMMONS, "NUMMONS" },
+    { NON_PM, "NON_PM" },
+    { LOW_PM, "LOW_PM" },
+    { SPECIAL_PM, "SPECIAL_PM" }
 };
 struct enum_dump objdump[] = {
 #include "objects.h"
@@ -1162,7 +1165,9 @@ dump_enums(void)
     for (i = 0; i < NUM_ENUM_DUMPS; ++ i) {
         raw_printf("enum %s = {", titles[i]);
         for (j = 0; j < szd[i]; ++j) {
-            nmprefix = (j == szd[i] - 1) ? "" : pfx[i]; /* "" or "PM_" */
+            int unprefixed_count = (i == monsters_enum) ? 4 : 1;
+            nmprefix = (j >= szd[i] - unprefixed_count)
+                           ? "" : pfx[i]; /* "" or "PM_" */
             nmwidth = 27 - (int) strlen(nmprefix); /* 27 or 24 */
             raw_printf("    %s%*s = %3d,",
                        nmprefix, -nmwidth, ed[i][j].nm, ed[i][j].val);