Temporary workaround for event handling serialization issue

Ludovic Rousseau pointed out that libusb_unlock_events() is not called
when a thread gets terminated with a signal, meaning that event waiters
will not be woken up in this case.

Add a temporary hack to libusb_event_handler_active() so that at least
the other threads will realise on the next iteration of their event
handling loop.

The real fix will likely involve reworking most of this.