systemd: merge master_staging_systemd with master

Right now we have the master_staging_systemd repository, see https://wiki.postmarketos.org/wiki/Systemd for usage.

Here's the plan for getting from there to having everything combined in the master branch:

1. Reduce the size of the diff as good as we can: all changes that work without having the extra-repos/systemd directory can already be made as merge requests against master. These would be about spitting openrc logic off into openrc-specific subpackages (that we do not install in the systemd-based installs), adding systemd services (with systemd subpackages), etc.

2. Wait for pmbootstrap#2328 (closed), so pmbootstrap and bpo are capable of splitting the extra-repos/systemd directory into a separate binary repository. This will take some time.

3. Incrementally create merge requests to get the systemd specific stuff into pmaports master. We probably don't need the entire (very much WIP) commit history there, maybe we could just get all involved authors with some shell foo and add them as Co-Developed-By: and then use one commit per package. I would make one merge request for all packages needed for pmbootstrap repo_bootstrap, and then make others afterwards until everything from the staging branch is in master.

4. Remove master_staging_systemd (note: once we do this, the staging binary repo automatically gets removed).

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Since 2. will take some time and cannot really be parallelized, help with 1. is appreciated. As well as testing the staging repository and fixing all bugs we find there, so ideally the systemd packaging completely works as expected when we reach 3.

Regarding timeline, let's try to get this done between v24.06 and v24.12.