Add support for server (remote) forwarded tcpip channels.
See RFC4254 Section 7.1
R=gustav.paul, jeff, agl, lieqiewang
CC=golang-dev
https://golang.org/cl/6038047
This removes the sole "exp/foo" import in the Go subrepos.
A separate CL will remove exp/terminal from the standard Go repository.
R=golang-dev, dave, r
CC=golang-dev
https://golang.org/cl/5966045