js-duk: Implement exit events

Replaces bind(K_BIND_SHUTDOWN, ...)

    function handler(ev)
    {
            switch (ev.type) {
            ...
            case "exit":
                    mrodder = ev.data;
                    if (mrodder)
                            mw.print("Your mw session was killed by " + mrodder);
                    break;
            ...
            }
    }
    mw.onevent.push(handler);