rest: add timestamp to shell/output

Provide a timestamp that tells when the command output from client was received.

src/client.c

@@ -205,6 +205,7 @@ static int og_resp_shell_run(struct og_client *cli, json_t *data)
 
 	free((void *)cli->shell.cmd);
 	free((void *)cli->shell.output);
+	cli->shell.tstamp = time(NULL);
 	cli->shell.cmd = strdup(cmd);
 	cli->shell.output = strdup(output);
 	cli->shell.retcode = retcode;

src/rest.c

@@ -813,6 +813,7 @@ static int og_cmd_run_get(json_t *element, struct og_msg_params *params,
 		}
 		json_object_set_new(object, "output", output);
 		json_object_set_new(object, "retcode", json_integer(cli->shell.retcode));
+		json_object_set_new(object, "tstamp", json_integer(cli->shell.tstamp));
 
 		json_array_append_new(array, object);
 	}

src/rest.h

@@ -75,6 +75,7 @@ struct og_client {
 		const char	*cmd;
 		const char	*output;
 		uint32_t	retcode;
+		time_t		tstamp;
 	} shell;
 };