Fix stdin handling in engine.Sender and engine.Receiver

This introduces a superficial change to the Beam API:

* `beam.SendPipe` is renamed to the more accurate `beam.SendRPipe`
* `beam.SendWPipe` is introduced as a mirror to `SendRPipe`

There is no other change in the beam API.

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

beam/router.go

@@ -78,7 +78,7 @@ func (route *Route) Tee(dst Sender) *Route {
 			return inner(payload, attachment)
 		}
 		// Setup the tee
-		w, err := SendPipe(dst, payload)
+		w, err := SendRPipe(dst, payload)
 		if err != nil {
 			return err
 		}