@GrahamcOfBorg test service-runner

I'm not very confident extending that perl-based systemd alternative should really be the way to go forward here, and assume many NixOS modules already rely quite heavily on systemd-provided features such as tmpfiles, config/state/runtime directory provisioning, dynamic user/groups and different service type behaviours - probably it'd make more sense for people to handle startup themselves?

I think @nlewo has experimented with something non-systemd as well, maybe he can provide some insights?

@flokli It's currently broken and blocking arion's update to 20.03. So unless something is terribly wrong with the implementation of the fix, I think it's an improvement.
Is it ok to debate the future of the script after this pr? Like many unsound things, it seems quite useful.

I also think we should move forward on this PR as it is since bringing an alternative implementation is another kind of task:/

Note I would be really interested in discussing on alternative implementation. Actually, I had already started to work on a s6 implementation (which is unfortunately no longer maintained).

S6 looks like a great alternative for lighter containers. I'm not sure if that will replace this script, because it is something quite different, but it is certainly a better fit something like arion.