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/examples/beamsh/builtins.go

@@ -272,7 +272,7 @@ func CmdPrint(args []string, stdout, stderr io.Writer, in beam.Receiver, out bea
 		}
 		// Skip commands
 		if a != nil && data.Message(payload).Get("cmd") == nil {
-			dup, err := beam.SendPipe(out, payload)
+			dup, err := beam.SendRPipe(out, payload)
 			if err != nil {
 				a.Close()
 				return