Initial attempt to establish a better integration between AI and ATC code.

Initial attempt to establish a better integration between AI and ATC code.
Various other patches that have been lingering around for a while:
 * Moved trafficcontrol.[ch]xx from the Airports directory to ATC, where
   it really belongs.
 * AI aircraft will request startup clearance, and ground control will
   approve.
 * Starting AI Aircraft will be pushed back to a predefined holding point
   on the ground network, and wait a while before taxiing out to the runway