Fred: do not give center zone higher priority than other zones

This was kind of useful for playing against the AI, but isn’t against
human players.
Fred attack rating: decrease leader worth

This just resulted in units suiciding themselves when they could get a
hit in.  This way, the leader is still considered twice as valuable as
normal units.
Fred: attack rating should take attack and counter attack into account

Only the attack rating was used so far for evaluation time reasons.
Now we also consider the counter attacks, but only for the first 50
attacks.  Will have to see how that works out in practice.
Retreat code: fix typo in math.max usage syntax
Retreat code: fix typo in array indices

Indices start at 1 in Lua.
Retreat code: fix bug in healing location determination

Healing locations are those next to healers, not the locations of the
healers.
Retreat code: fix occasional bug due to terrain defense penalty rating

The terrain penalty must be smaller than the heal_score rating, which
sometimes can be as small as 1.  This sometimes lead to units with 1 HP
to retreat to mountains rather than to villages.
Retreat code: fix bug in curing score evaluation

Don’t use the curing score of an ability if another ability with a
higher score was already found previously.