Fix resurrection with Arch-viles

Enemies will now keep their stats upon death. They will only give up their ID if
they somehow disappear, like a lost soul hitting the end of its' death animation
or Thing_Remove being nasty.

When a monster dies, it's flagged as NOXP and NODROPS so re-killing it will
yield nothing, therefore it's not possible to gimp missions or auras to give you
tons of credits or things like that.

This should hopefully work better now, but it needs testing.