disclosing genocided/extinct monsters

Make the handling of unique monsters consistent between vanquished
monsters and genocided/extinct monsters.  No visible difference to
players.

This also prevents Nazgul and erinys from being polymorphed into
some other form to reduce the chance that their kill count fails
to match the expected number when they're reported to be extinct.
[My long test game (with 3451 total dead critters as of the last
save file) used for exercising the sorting of vanquished monsters
included
 120 soldiers
 111 wolves
   9 Nazgul
   2 erinyes
and the genocided/extinct list had none genocided, those four
extinct.  No doubt the missing third erinys was alive somewhere
rather than counted as something else after getting polymorphed,
so this band-aid wouldn't have helped this particular game.]

src/mon.c

@@ -3217,8 +3217,14 @@ boolean msg;      /* "The oldmon turns into a newmon!" */
 
     /* Riders are immune to polymorph and green slime
        (but apparent Rider might actually be a doppelganger) */
-    if (is_rider(mtmp->data) && mtmp->cham == NON_PM)
-        return 0;
+    if (mtmp->cham == NON_PM) { /* not a shapechanger */
+        if (is_rider(olddata))
+            return 0;
+        /* make Nazgul and erinyes immune too, to reduce chance of
+           anomalous extinction feedback during final disclsoure */
+        if (mbirth_limit(monsndx(olddata)) < MAXMONNO)
+            return 0;
+    }
 
     if (msg) {
         /* like Monnam() but never mention saddle */