If a Pipe method is called, return the underlying
reader/writer from session.clientChan, bypassing the
io.Copy and io.Pipe harness.

StdoutPipe and StderrPipe now return an io.Reader not
an io.ReadCloser as SSH cannot signal the close of the
local reader to the remote process.

R=rsc, agl, gustav.paul, cw
CC=golang-dev
https://golang.org/cl/5493047