Implement counter-on-miss for Guard

This is much more fairly balanced--and less prone to abuse--than
allowing automatic counterattacks against all melee attacks.  Guard
raises your evasion slightly against melee attacks, and only if the
attack misses do you get to counterattack.

Some more gamedef refactoring was done here, as well, and decided on a
battle theme for the #11 battle in Spectacles III.