Add logging to show key exchange algorithm negotiated

[kevinmkane]

It's not obvious what algorithm is being used for key exchange on the control connection. Add logging to state which algorithm is being used when a tunnel is established.

[kevinmkane]

There isn't a great API for doing this. What I've done is printed the first NID in the list of shared groups computed by the server, and this is the group that ends up getting used. Unfortunately this is only available on the server, so we can only print the group_id on the server. There's no good API for doing this on the client, as the place where this data is stored is not publicly exposed.