Adds nether spawns to corpse mapextras

Maybe they killed 'em, maybe they're just curious, but either way these
ought to liven up the ol' "free loot" arguments.

This doesn't affect any strings, and it shouldn't break anything.  So it
should be safe to go in, but I wouldn't call this a bugfix.

(Unless you're hooked on mi-go or something.  ;-) )
Merge pull request #6379 from KA101/CorpseKillers

Adds nether spawns to corpse mapextras