Allow an undetermined turnout at the beginning of a route

Allow an undetermined turnout at the beginning of a route

If route plans are regenerated when the train's last vehicle is on the
sensor following a turnout in a points-facing movement, the first track
of the lead route is a turnout with undetermined path.  Route_changed
would then try to figure out which route the next allocation will occur
on and fail at the very beginning because advance_to_track refused to
use the undetermined turnout.