Added requested server convars:
- trainskeeprunning: If true, trains don't automatically stop when there's no-one on them. Default: false.
- stop_all_trains: Stops the engines of all train engines on the server.
Trains can now detect if static (as in non-moving, not necessarily actual static) obstacles that they've backed off from have been destroyed, allowing the train to continue. Previously this required the train to reverse direction momentarily first.
New system for junction selection, gives correct left/right track selection at any junction. Also made sure the main rail loop is always the default selection at a siding junction, even if the siding is actually the straightest route.