x/crypto/ssh/agent: Support v1 remove all message

Some ssh-agent clients expect the server to support remove all messages
for protocols 1 & 2 and error if protocol 1 support is missing.

This adds a null-op implementation of the remove all message in similar
fashion to the existing list all message support.

Fixes golang/go#15159

Change-Id: I9389885d89c9147f3e10850893bba6ed7d0a4e82
Reviewed-on: https://go-review.googlesource.com/21468
Reviewed-by: Han-Wen Nienhuys <hanwen@google.com>
Run-TryBot: Han-Wen Nienhuys <hanwen@google.com>

ssh/agent/client.go

@@ -76,7 +76,8 @@ type AddedKey struct {
 
 // See [PROTOCOL.agent], section 3.
 const (
-	agentRequestV1Identities = 1
+	agentRequestV1Identities   = 1
+	agentRemoveAllV1Identities = 9
 
 	// 3.2 Requests from client to agent for protocol 2 key operations
 	agentAddIdentity         = 17
@@ -376,6 +377,8 @@ func unmarshal(packet []byte) (interface{}, error) {
 		msg = new(identitiesAnswerAgentMsg)
 	case agentSignResponse:
 		msg = new(signResponseAgentMsg)
+	case agentV1IdentitiesAnswer:
+		msg = new(agentV1IdentityMsg)
 	default:
 		return nil, fmt.Errorf("agent: unknown type tag %d", packet[0])
 	}