daemon: avoid race when quitting workers

daemon: avoid race when quitting workers

While the master process has a self-pipe to avoid missing
signals, worker processes lack that aside from a pipe to
detect master death.

That pipe doesn't exist when there's no master process,
so it's possible DS::close never finishes because it
never woke up from epoll_wait.  So create a pipe on
the worker_quit signal and force it into epoll/kevent
so it wakes up right away.