systemd: switch from our own fork to upstream repo + local patches

After patching, this produces exactly the same source code as in our
custom fork, but having the actual patches inlined inside nixpkgs makes
it easier to get rid of them.

In case more complicated rebasing is necessary, maintainers can

 - Clone the upstream systemd/systemd[-stable] repo
 - Checkout the current rev mentioned in src
 - Apply the patches from this folder via `git am 00*.patch`
 - Rebase the repo on top of a new version
 - Export the patch series via `git format-patch $newVersion`
 - Update the patches = [ … ] attribute (if necessary)