Change cigotuvi's shed behaviour

Rather than a complex formula based on spellpower & # of corpses,
it's now a less complex formula based purely on # of corpses.

Ref the comment on player::maybe_degrade_bone_armour().
Change Cigotuvi's armour ratios

Now gives (power / 100 + 0.5) AC+SH per corpse. (So at power 0,
1 AC+SH for every 2 corpses; at 50, 1 per corpse, and at 100,
1.5 per corpse.)

Should give more reasonable results at low & high power.

Spellpower is now capped at 100.