go.crypto/ssh: add support for remote tcpip forwarding

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

ssh/messages.go

@@ -139,7 +139,7 @@ type channelOpenConfirmMsg struct {
 // See RFC 4254, section 5.1.
 type channelOpenFailureMsg struct {
 	PeersId  uint32
-	Reason   uint32
+	Reason   RejectionReason
 	Message  string
 	Language string
 }