Bugfixes and feature improvements related to AI auto flightplan generation.

Bugfixes and feature improvements related to AI auto flightplan generation.
When AIFlightPlanCreateTaxi() function is called with the firstFlight
argument set to true, this is supposed to handle situations where the
the aircraft's timetable indicates it should have left between about 5 to
20 minutes earlier. In the previous version, all these aircraft started
taxiing from the first parking location available in the network, due
to the fact that the variable gateId was not assigned a value. In this
patch, route tracing starts from an assigned gate and the network node
following code is fast forwarded to a random location along the taxiways
to give a more realistic and natural distribution of taxiing aircraft
after startup.
    This patch further addresses some weird ballet-dancing behavior that
aircraft were showing just prior to/right after parking and which was
related to a number of more or less duplicate waypoints in the transition
from createTaxi() to createParking() to createPushBack() to createTaxi().
    Finally, a blatant typing error in the getParking() function was fixed.