Align NUT systemd units with recommended behavior (enabled to start == running even if not configured) #837
Description
Following up from discussion in #766, we can offer some tricks for either auto-disabling NUT units from auto-starting in systemd if there is no configuration (to avoid using system resources), or allow startup with ALLOW_NO_DEVICE=true setting (for upsd) so it would run and report that it sees zero device config sections.
Arguably, this is more of a distribution maintainer headache than NUT's to integrate a package with distro policies du-jour, but we might help that with sample code or documentation hints, at least.