Fred: add zone specific functions for advancing units within a zone
Fred: change hp_ratio threshold for center zone
Fred: reconsider location for holding if best hex is unthreatened

In that case, terrain etc. doesn’t matter and we might as well advance
to the most forward unthreatened hex.
Fred: change how zone_enemies are selected

Use enemies that can reach a certain key hex, rather than all enemies
within a certain area.
Zone: change how advancing toward a zone is done

Use the new zone_advance_rating function.
Fred: use leader_distance_map, not path_map to determine up/downstream direction
Fred: remove now unused corridor and path maps
Fred zone holding: give enemies full moves before calculating their reach
Fred: minor changes to debug output