Adjust corpse armour

Remove invisible cap on SH/AC; instead adjust odds for bits
dropping off. See comments in maybe_degrade_bone_armour().

Also, make the spell deterministic in how much SH/AC you gain, to
avoid encouraging weird degenerate shit involving scooping up
lone corpses seperately.
Fix indentation
Add monster Cigotuvi's Embrace (Hangedman)

To hellwings, which start with it active.

+(6 + hd/3) AC & SH (single-leve); for hellwings, that's +8 of
each. 1/4 chance of falling off every time they're hit or block.
Clarify a message