Implement STUN to avoid relaying all packets in case of client-side NAT

[Arkanosis]

If there's a NAT not only server-side, but client-side as well, rssht currently works by ssh-ing to a relay server from the server, then ssh-ing again to the same relay server from the client. This works well, but it introduces load on the relay server and impacts both latency and bandwidth.

Implementing STUN / ICE would enable direct SSH connection from the client to the server, by just signalling the server to the client and vice-versa. In the rare cases where it wouldn't work, the current TURN approach could be used as a fallback.

Additionally, it might be possible to rely on some public STUN server to avoid the public server requirement altogether.

Also, see #11 for an approach which doesn't even need a STUN server (pwnat).