Improved selection of active runway (currently AI traffic only).

Improved selection of active runway (currently AI traffic only).
The original code selected the most preferable runway combination,
regardless of which combination was currently active. This patch
builds in some additional resistance against change, by keeping
track of which runways are already in use, and forces a change
in active runways only when new (wheather/time of day) conditions
force it to do so, resulting in much more consistent runway assignment.
behavior.