Kill the udev monitor thread on exit.

Index: udev-monitor.c
--- udev-monitor.c.orig
+++ udev-monitor.c
@@ -334,6 +334,7 @@ udev_monitor_unref(struct udev_monitor *um)
 	TRC("(%p) refcount=%d", um, um->refcount);
 	if (--um->refcount == 0) {
 		close(um->fds[0]);
+		pthread_cancel(um->thread);
 		pthread_join(um->thread, NULL);
 		close(um->fds[1]);
 		udev_filter_free(&um->filters);