crypto/ssh: allow identities to be constrained.

The ssh-agent protocol allows the usage of keys and certs added to a
given agent to be constrained in certain ways. The only constraints
currently supported are lifetime (keys expire after some number of
seconds) and confirmation (the agent requires user confirmation before
performing any operations with the private key).

Change-Id: Idba5760db929805bf3da43fdcaca53ae6c479ca4
Reviewed-on: https://go-review.googlesource.com/12260
Reviewed-by: Adam Langley <agl@golang.org>
Run-TryBot: Adam Langley <agl@golang.org>
Reviewed-by: Peter Moody <pmoody@uber.com>

ssh/test/agent_unix_test.go

@@ -21,7 +21,16 @@ func TestAgentForward(t *testing.T) {
 	defer conn.Close()
 
 	keyring := agent.NewKeyring()
-	keyring.Add(testPrivateKeys["dsa"], nil, "")
+	if err := keyring.Add(agent.AddedKey{PrivateKey: testPrivateKeys["dsa"]}); err != nil {
+		t.Fatalf("Error adding key: %s", err)
+	}
+	if err := keyring.Add(agent.AddedKey{
+		PrivateKey:       testPrivateKeys["dsa"],
+		ConfirmBeforeUse: true,
+		LifetimeSecs:     3600,
+	}); err != nil {
+		t.Fatalf("Error adding key with constraints: %s", err)
+	}
 	pub := testPublicKeys["dsa"]
 
 	sess, err := conn.NewSession()