zugfuhrer: add experimental live broadcast mechanism

- Interested parties can now connect to `tspl-zugfuhrer` over TCP
  to receive a live stream of train activation and movement events.
- (On connect, you also get a dump of all trains currently running,
  to help you out.)
- This will imminently be used to implement a nice fancy web UI
  with a bunch of pretty train movements flying by. Watch this space!