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.

include/decl.h

@@ -972,6 +972,7 @@ struct instance_globals {
     /* mon.c */
     boolean vamp_rise_msg;
     boolean disintegested;
+    boolean zombify;
     short *animal_list; /* list of PM values for animal monsters */
     int animal_list_count;