Improve argument parsing for createViaTo, createViaFromTo to
handle Airway ghosts as well as strings. As part of this, allow
specification of the airway level explicitly when looking
up an airway.
SF-ID: https://sourceforge.net/p/flightgear/codetickets/2686/
Both the route-path cade and the RNAV code were using some bad logic
to compute the intersection point. All fixed now, but requires a
new helper in Simgear.
One test so far, attempting to reproduce a bug I see with the 737.
Note these tests are skipped unless you have procedures available
and set a magic env var to find them.