udev: do not wait for event queue being empty on exit

udev: do not wait for event queue being empty on exit

When the manager process is requested to terminate, if a worker process
try to lock a block device and failed, then the worker returns a
TRY_AGAIN notification and the event is requeued. Hence, the event queue
may have pending events even after manager_exit() is called. In such
situation, sd_event_exit() will never called, and udevd will stuck.

This makes, after termination is requested, it only checks whether there
are any events currently being processed.