Use Mono.Nat for UPnP port forwarding.

https://github.com/mono/Mono.Nat is a MIT licensed library created for http://www.monotorrent.com/ I guess it is superior to the code snippet used before. Tested with AVM Fritz!Box. It never crashes the game (tried UPnP disabled, UPnP port forwarding disabled and UPnP port forwarding set to allowed at router) and forwards both external and internal port now. It also searches for a device only once at game start while loading the mounting the files and loading the shellmap (which takes some time) so it does not slow down the lobby entering when creating a server.