Make item::bday a signed int to avoid underflow errors on new games.

Should fix #2864.
Looks like if turn was > bday of a zombie corpse, the number underflowed to MAXINT-ish, and the zombie immediately resurrected.  Therefore on any playthrough with unpulped zombie corpse piles, the whole pile would try to get up at once.
Might also want to make revived zombies try to spawn elsewhere instead of just failing to spawn, but that could lead to other issues.