ssh: support forwarding of Unix domain socket connections

This commit implements OpenSSH streamlocal extension, providing the equivalent
of `ssh -L local.sock:remote.sock`.

Change-Id: Idd6287d5a5669c643132bba770c3b4194615e84d
Reviewed-on: https://go-review.googlesource.com/38614
Reviewed-by: Han-Wen Nienhuys <hanwen@google.com>
Run-TryBot: Han-Wen Nienhuys <hanwen@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>