Edit zig loot mildly.

 * Add an average of one restore abilities potion per zig (in initial
   zig floor settings), since Tomb floors and extended mana channeling
   / summons can eat away at one's stats easily and it's nice to be
   nice about it.
 * Add a tiny chance of beneficial mutation in the best of the zig
   loot, because beneficial mutation is cool. The tiniest of odds
   are cut from potions of experience, cure mutation, and royal
   jelly stacks to slot it in.