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/server.go

@@ -192,12 +192,6 @@ func (s *connection) serverHandshake(config *ServerConfig) (*Permissions, error)
 		return nil, err
 	}
 
-	if packet, err := s.transport.readPacket(); err != nil {
-		return nil, err
-	} else if packet[0] != msgNewKeys {
-		return nil, unexpectedMessageError(msgNewKeys, packet[0])
-	}
-
 	// We just did the key change, so the session ID is established.
 	s.sessionID = s.transport.getSessionID()