fix github issue #109 - healing spells

fix github issue #109 - healing spells

Fixes #109

Spells of healing and extra healing cast at monsters were handling
monster blindness differently from other forms of healing.  (Potions
also work differently when drunk by monsters but I haven't changed
that since it seems to be intentional.)

Hero:
   potion of healing cures blindness if blessed; spell of healing
      cast at skilled or better now behaves likewise;
   potion of extra healing cures blindness if not cursed; spell of
      extra healing is inherently not cursed and already behaved
      likewise;
   potion of full healing always cures blindness even if cursed.

Monsters quaffing potions:
   plain healing cures blindness if not cursed;
   extra healing and full healing always cure blindess.

Hero casting healing spell at monster:
   plain healing behaves like the hero plain healing case:  cures
      blindness as if blessed when cast at skilled or expert level;
      this is a change in hehavior--it used to cure timed blindness
      even if unskilled and not cure 'permanent' blindness at all;
   extra healing cast by hero is inherently not cursed so always
      cures blindness.