Balance fixes to monster/monster zombie creation attacks

These were unbalancing the game a) in the Castle and b) if they
woke up unique monsters (most notably the Wizard of Yendor).

I considered adding a difficulty check, but this commit instead
just directly fixes the symptoms. (It doesn't make sense for the
Castle to contain a monster that would kill or be killed by its
inhabitants: they should have died long before the hero arrived.
So for liches/zombies to exist in the Castle at all, there must be
a truce.)

src/mon.c

@@ -2347,9 +2347,21 @@ mfndpos(
 staticfn long
 mm_2way_aggression(struct monst *magr, struct monst *mdef)
 {
-    /* zombies vs things that can be zombified */
+    /* liches/zombies vs things that can be zombified
+
+       Note: avoid this on the Castle level, partly for balance reasons
+       (the monster-versus-monster fights clear out significant portions of
+       the Castle and make it easier than it should be), partly for flavor
+       reasons (monsters who attacked other monsters to zombify them would
+       have been counterattacked to death long before the hero arried).
+
+       Also don't include unique monsters in this, otherwise it leads to
+       them waking up early (e.g. because a zombie decided to attack the
+       Wizard of Yendor). */
     if (zombie_maker(magr) && zombie_form(mdef->data) != NON_PM)
-        return (ALLOW_M | ALLOW_TM);
+        if (!Is_stronghold(&u.uz) &&
+            !unique_corpstat(magr->data) && !unique_corpstat(mdef->data))
+            return (ALLOW_M | ALLOW_TM);
 
     return 0;
 }