Helping out by merging/solving conflicts in the open MRs

[dadinn]

This PR has exactly the same content as https://github.com/holomorph/systemd-mode/pull/21.

The only difference here is that the merged commits have been rebased and fast-forwarded instead, to make the main branch look cleaner, and not littered with merge commits.

I've ensured the commit information is preserved as much as possible, such that the work from @abougouffa to merge the features is correctly addressed and dated.

Refs

https://github.com/holomorph/systemd-mode/pull/11 https://github.com/holomorph/systemd-mode/pull/15 https://github.com/holomorph/systemd-mode/pull/18 https://github.com/holomorph/systemd-mode/pull/19

[abougouffa]

Great! Thank you @dadinn, I hope this gets merged soon.