charon-cmd: Add --agent option to authenticate using ssh-agent(1)

The socket path is read from the SSH_AUTH_SOCK environment variable.
So using this with sudo might require the -E command line (or an appropriate
sudoers config) to preserve the environment.

src/charon-cmd/cmd/cmd_connection.c

@@ -387,6 +387,7 @@ METHOD(cmd_connection_t, handle, bool,
 			this->identity = arg;
 			break;
 		case CMD_OPT_RSA:
+		case CMD_OPT_AGENT:
 			this->key_seen = TRUE;
 			break;
 		case CMD_OPT_LOCAL_TS: