Provide a reliable mechanism for terminating a background worker.

Although previously-introduced APIs allow the process that registers a
background worker to obtain the worker's PID, there's no way to prevent
a worker that is not currently running from being restarted.  This
patch introduces a new API TerminateBackgroundWorker() that prevents
the background worker from being restarted, terminates it if it is
currently running, and causes it to be unregistered if or when it is
not running.

Patch by me.  Review by Michael Paquier and KaiGai Kohei.