Uploading the patch here per elexis' request (https://trac.wildfiregames.com/ticket/2305#comment:40), but still proceeding to work on it.
Current state: the patch works with some network configurations, see details here: https://trac.wildfiregames.com/ticket/2305#comment:39
TODO:
- re-test with multiple 0ad instances on the same machine (host + client and multiple clients) (previously crashed, but should be fixed now)
- split STK-based code into a separate file
- change the STUN-server setting (once our own server would be configured)