pdist-bootcheck: Run async update as transient service

Run the update as a separate service instead of as a forked
process to avoid the error message

    pdist-bootcheck.service: Failed to destroy cgroup /system.slice/pdist-bootcheck.service, ignoring: Device or resource busy

and to capture the exist status in systemd and the output in the
journal.

pdist/pdist-bootcheck

@@ -26,7 +26,8 @@ else
 fi
 
 if [ "$need_async_pdist" ]; then
-    netcat $(distmaster) 237 >/dev/null 2>&1 </dev/null &
+    systemd-run --unit pdist-async-update --service-type=simple --remain-after-exit \
+        netcat $(distmaster) 237
     echo triggered background update
 elif [ "$need_pdist_and_reboot" ]; then
     if [[ -e /var/run/updatecheck.reboot-triggered ]]; then