Systemd support in ARCHED
The HED now reports it's status (ready/failure) and PID directly to systemd via notification mechanism (if build with systemd support).
As PID will be known to systemd for sure in this case, A-REX unit will run HED in daemon mode, preserving all features including watchdog.
This approach also solves BUGZ-3828.
P.S. This approach also opens another future modifications, e.g. possible socket activation of HED.