battle_calcs.best_defense_map: add optional ignore_these_units parameter

Units listed in this parameter will be taken off the map for route
finding.
Fred zone holding: ignore own units with MP left for enemy defense map

Previously, own units with MP left counted as blocking the path for
enemy units.  This had, for example, the curious side effect that an AI
unit might move off a village, because that village was considered
unreachable by the enemy.