ssh/agent: use right message to unlock agent, with related integration tests

Change-Id: Ie2e7618be63179fb65b8eea60684254712149a77
Reviewed-on: https://go-review.googlesource.com/50810
Reviewed-by: Han-Wen Nienhuys <hanwen@google.com>
Run-TryBot: Han-Wen Nienhuys <hanwen@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

ssh/agent/server.go

@@ -106,7 +106,7 @@ func (s *server) processRequest(data []byte) (interface{}, error) {
 		return nil, s.agent.Lock(req.Passphrase)
 
 	case agentUnlock:
-		var req agentLockMsg
+		var req agentUnlockMsg
 		if err := ssh.Unmarshal(data, &req); err != nil {
 			return nil, err
 		}