Add balrugs to gehenna, blizzard demons to cocytus.

Surprisingly, they weren't native monsters to either branch. Balrugs could
be found in Gehenna, but only in vaults. They should generate normally now,
although you won't find more than one or two per level, usually.

The cocytus ending vaults still don't have blizzard demons.

Conflicts:
	crawl-ref/source/mon-pick.cc