Multiple connection support for GUI clients

[Jason A. Donenfeld]

On iOS and Android, this is a limitation of the APIs provided to us by
Apple and Google. On macOS, the API is extremely buggy with multiple
tunnels, and we're not sure we can make it usable until Apple
straightens out the behavior there in a future release. If you'd like
to poke around with the latter, feel free to join us in #wireguard on
Freenode.