fix: 2nd restart of bgprocess service hangs

Because stop_issued is not reset when a bgprocess service stops, the
next time we try to stop the service we don't actually signal it. This
causes a hang (including at shutdown).

src/proc-service.cc

@@ -550,6 +550,7 @@ void bgproc_service::handle_exit_status() noexcept
         // We won't log a non-zero exit status or termination due to signal here -
         // we assume that the process died because we signalled it.
         if (stop_pid == -1 && !waiting_for_execstat) {
+            stop_issued = false;
             stopped();
         }
     }