x/crypto/ssh: hide msgNewKeys in the transport layer.

This ensures that extraneous key exchanges cannot confuse application
level code.

Change-Id: I1a333e2b7b46f1e484406a79db7a949294e79c6d
Reviewed-on: https://go-review.googlesource.com/22417
Reviewed-by: Han-Wen Nienhuys <hanwen@google.com>
Run-TryBot: Han-Wen Nienhuys <hanwen@google.com>
Reviewed-by: Adam Langley <agl@golang.org>

ssh/mux.go

@@ -227,9 +227,6 @@ func (m *mux) onePacket() error {
 	}
 
 	switch packet[0] {
-	case msgNewKeys:
-		// Ignore notification of key change.
-		return nil
 	case msgChannelOpen:
 		return m.handleChannelOpen(packet)
 	case msgGlobalRequest, msgRequestSuccess, msgRequestFailure: