abuild: add systemd splitfunc
The reason for this MR is to simplify our development downstream. It has nothing to be with use in alpine, so I would hope it does not come as too controversial. I certainly have the wonder of whether this kind of alpine tooling is expected by the maintainers to only be used by alpine, and whether contributions for downstream use are welcomed. I'd hope it's fine to carry this here, as we really don't want to have to carry a patched version forever, and neither do we want to write an alternative.