Extract corpse-revival code to a method of game
Zombies rise from the dead after a certain amount of time
Fix some revification bugs
Save state of zombie death drops
Fix zombie revival timing - no longer tuned for testing

Whoops, left the code in a testing state when I committed at some point.
Add a message when nearby corpses rez
Merge remote-tracking branch 'dda/pr/1020' into dda-master