Fred: change zone id for cfg_rush_right
Fred: change unit filters for the three main corridor zones

This is now possible since the rush on the right counts as a separate
zone from the right corridor.
Fred: add key_hexes parameter to zone cfgs

This is needed because we want distances from several key hexes for the
center zone, not just to the hold hex, in order to get the balancing
between zones right.
Fred: add a zone priority rating to the cfgs
Fred hp_ratio(): add optional parameter @weights for enemy HP

This allows us, for example, to weight enemies farther away less
strongly than closer enemies.
Fred village grabbing: make evaluation more efficient and much faster

This was coded incredibly inefficiently …
Fred: use print_time() instead of print() for some debug messages
Fred zone holding: use weights to get HP and unit ratios

Enemies farther away don’t count as much as close enemies.