Fix for js.time_limit stuck at 0

When setting js.time_limit in mirc-client.js the value sticks throughout the session, which means any other scripts run will also have js.time_limit = 0. This modification stores the value of js.time_limit upon invocation and then resets it when the main loop terminates. While this should be sufficient in most cases, it obviously won't catch something due to an error. These should be rare if not non-existent now.