Passive zombie revival

Add in basic zombie revival, which occurs after a 6-hour grace period. Only unbutchered zombie corpses will revive.

The only difference so far between revived and naturally-spawned zombies is that revived zombies don't drop any loot (since it should have all been dropped when they were killed the first time). Some sort of long-term change would be desirable, though.

There's naturally a lot of room to improve the system, but this is a functional first pass to let us shake out any bugs (such as segfaulting if a corpse rezzes while being butchered - I think I fixed that one...).