Fred zone attack: take XP into account for counter attack evaluation
ai_helper.get_attack_combos: bug fix: a hex does not count as blocked by the attacker unit itself

This takes care of the case when a unit has no moves but attacks left,
which was previously excluded.
Fred zone attack: include units that have no moves but attacks left in attack evaluations

Previously that was excluded because units with no moves were used for
a non-attack move already and therefore likely should not attack.  But
there are exceptions.
Fred zone attack: counter attack calculation does not make sense for units with MP=0

... since they cannot move away anyway.