Some tweaks and finetuning made possible by the multihreaded modelloader

Some tweaks and finetuning made possible by the multihreaded modelloader
and the pushback code:
 - Traffic manager starts modelload requests immediately upon program
   loading
 - Only create legs 1 (push back) or five (cruise) of AI traffic.
 - AIAircraft's rather obsessive behavior to circle around a waypoint is
   largely resolved
 - More realistic ground steering for AI aircraft.