Beam: allow sending messages without attachments.

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
Beam: change the prototype of SendPipe() to return a *net.UnixSocket

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)