Out-of-band data + SSH Agent forwarding

Pluggable out of band communication mechanism over Mosh transport
layer and agent forwarding support in top of out of band mechanism.

I contrubute this code to Mosh project under the same license that
Mosh itself is distributed. All licensing options and clauses
included.

Signed-off-by: Timo J. Rinne <tri@iki.fi>