Lua AIs: Remove unnecessary arguments in call to wesnoth.put_unit()
ai_helper: improve criterion for impassable terrain

Rather than checking whether the movement cost is <99, check whether it
is <= max_moves for the unit.
Retreat AI eval: poisoned units do not get resting healing bonus

... in evaluation of best retreat positions.
ai_helper.get_enemy_dst_src: add optional parameter 'enemies'

If 'enemies' table is given, use it, otherwise use all enemy units.
ai_helper.get_dst_src(units): minor code simplification