x/crypto/ssh: if debugMux is set, also log global messages.

Change-Id: I685ba7f30f1d2e2bc3c078c7bddb6d383a179113
Reviewed-on: https://go-review.googlesource.com/22416
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Han-Wen Nienhuys <hanwen@google.com>

ssh/mux.go

@@ -131,6 +131,9 @@ func newMux(p packetConn) *mux {
 
 func (m *mux) sendMessage(msg interface{}) error {
 	p := Marshal(msg)
+	if debugMux {
+		log.Printf("send global(%d): %#v", m.chanList.offset, msg)
+	}
 	return m.conn.writePacket(p)
 }