Fred: reduce priority of center zone
Fred: cannot use enemy_turn_maps to determine relative priorities of zones

All AI units need to be ignored for this, and enemy_turn_maps only
ignores those with MP left.
Fred zone holding: fix bug in rating of hexes with enemy units on them

Hexes with enemy units on them are considered in the adjacent hex
analysis.  There was previously a division by zero problem for them,
causing such strange effects as partially surrounded AI units stepping
one hex off a village (because the village got the invalid 1/0 rating).