Fred zone hold: streamline moving a unit toward a zone goal

If none of the hexes the unit can reach are threatened by enemies, move
the unit directly toward the goal hex of the zone.  This avoids, among
other things, having it stop on favorable terrain without using its
full movement range.
Fred zone hold: make terrain defense rating additive contribution

It was simply too strong a modifier when set up as a multiplicative
factor.
Fred zone hold: increase importance of forward position rating

This is a rush AI after all.
Fred zone hold: include hexes occupied by own units with MP

Or more correctly, own units that can move out of the way.  It is
possible for a unit to have MP left, but not be able to move anyway.