systemd preparation

There's still some more work needed for systemd, so to save on annoying rebase conflict resolution let's get some of the preparatory work in.

Pushing to this MR

To keep things organised, the main systemd MR !4894 (closed) is actually against the systemd-prep branch, this way the commits from here don't show up in that MR as well. This sounds like a rebasing nightmare but it's actually quite simple with rebase.updateRefs, for those unfamiliar it works like this:

1. do git config rebase.updateRefs true
2. checkout the systemd-prep branch and git pull
3. checkout the systemd branch and git pull
4. do all your changes on the systemd branch, make fixup commits and do an interactive rebase
5. push both branch when done

When doing the interactive rebase you'll see a new update-ref line, this tells git to update the ref that the systemd-prep branch name refers to. So in this way the two branches will remain in-sync and we don't have commits duplicated across MRs.

mentioned in merge request !5563 (merged)