Mild Zombie Apocalypse

When a zombie (or lich) kills a monster in melee without a weapon,
the monster can rise few turns later as a zombie.

The only creatures that can be zombified are ones that actually have
a zombie counterpart monster. A zombie cannot turn a jackal into
a zombie, for instance. But it could turn a shopkeeper into a human
zombie, or a dwarf king into a dwarf zombie.

Zombies will fight with monsters that can be turned into zombies.

Originally this was a SliceHack feature, but this is based on xNetHack
version of it, with some modifications.

src/decl.c

@@ -498,6 +498,7 @@ const struct instance_globals g_init = {
     /* mon.c */
     UNDEFINED_VALUE, /* vamp_rise_msg */
     UNDEFINED_VALUE, /* disintegested */
+    UNDEFINED_VALUE, /* zombify */
     NULL, /* animal_list */
     UNDEFINED_VALUE, /* animal_list_count */