Thomas Foerster: Replaced AI network route tracing algorithm by a much more

Thomas Foerster: Replaced AI network route tracing algorithm by a much more
efficiently performing "Dijkstra algorithm".

Durk Talsma: Added the detection of "circular" wait situations in the AI
ground network. A circular wait is a situation where aircraft a waits for
b; b waits for c; and c (in turn) waits for a. The checkCircularWaits
function detects these situations.

The current "solution" to a circular wait is rather crude: Remove the
aircraft from the scene. A proper solution needs a lot more work, however,
and at least this patch stops the AI system from clogging up. in case of a
circular wait.