battle_calcs.lua: fix a variable name
Fred village grab: take villages from the back first

to eliminate back and forth movement inside a zone
battle_calcs.lua: fix bug in get_attack_map_unit() for enemy units
Fix typo in variable name
ai_helper.get_live_units: don't use filter_wml

It’s slow, the new method is between 10 and 20 times faster.
ai_helper: do not use empty tables as filters

It’s faster not to provide any argument to wesnoth.get_units() if all
units are to be found.
Fred: add a zone prioritization scheme
Fred: don't use formula= in filters

It’s slow.
Fred: don't start full offensive mode until Turn 9

Eventually we’ll get rid of this anyway.
Fred: fix bug in caching index for attacks

This led to the wrong attacks sometimes being pulled out of the cache
and thus some of the dubious attack decisions Fred has made.