Purple worm changes

Shriekers only spawn purple worms when they're appropriate difficulty.
Non-tame Purple worms eat corpses off the ground.
Baby purple worms attack shriekers.
Hero polyed into baby purple worm is warned against shriekers.

Original changes by copperwater <aosdict@gmail.com>, added with some
formatting adjustments and consolidation.

src/monmove.c

@@ -1534,6 +1534,13 @@ register int after;
                     return 2; /* it died */
             }
 
+            /* Maybe a purple worm ate a corpse */
+            if (ptr == &mons[PM_PURPLE_WORM]
+                || ptr == &mons[PM_BABY_PURPLE_WORM]) {
+                if (meatcorpse(mtmp) == 2)
+                    return 2; /* it died */
+            }
+
             if (!*in_rooms(mtmp->mx, mtmp->my, SHOPBASE) || !rn2(25)) {
                 boolean picked = FALSE;