ssh: add debug print at the lowest level

This is a simple minded, fast print, suitable for debugging timing
sensitive issues.

Change-Id: I9ae3df5fe86f1883c1fa9265b6f7f9a98d33747e
Reviewed-on: https://go-review.googlesource.com/36054
Run-TryBot: Han-Wen Nienhuys <hanwen@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

ssh/mux_test.go

@@ -499,4 +499,7 @@ func TestDebug(t *testing.T) {
 	if debugHandshake {
 		t.Error("handshake debug switched on")
 	}
+	if debugTransport {
+		t.Error("transport debug switched on")
+	}
 }