Improve systemd units

[calestyo]

Hey.

Some ideas…

Not so sure about the last commit, since I haven’t found why you had put AccuracySec in at the first place. So if there was a good reason for it, just skip that commit.

Thanks, Chris.

[zwimer]

Regarding ConditionPathExists=. If I understand correctly: Personally, I think the service should fail if the config does not exist. Missing a config is a bad state and I'd rather be alerted than assume my system is backing up as usual because it decided not to run and thus not to raise and errors. I can always systemctl stop btrbk if I dislike the warnings. This is especially the case if the user has added an OnFailure=notify-failed@%n trigger, or other triggers, which not-failing might not trigger.

That is, my backup system is a 'set it and forget it' type of program for me, so if the config suddenly disappears I likely wouldn't notice anything went wrong unless systemd notifies me of services failing, which it sounds like this would prevent? I imagine that is true for other people as well so I'm not sure if this is a 'good default' to push on people.

If AssertPathExists instantly fails the btrbk service rather than trying to actually run it, I think that's a better default here.