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.