NAT-T support should also be added

[rus-kilian]

Since not all networks are available on public IP, NAT-T would be a great addition.

Also, this would pave the path towards adding full roadwarrior/client support (via StrongSWAN).

[eugeneia]

This implies encapsulating ESP streams over UDP and managing the port fields accordingly. The former is rather trivial while the latter might need some state matched to the SA. Needs some upfront design work on how to model this in Vita.