Draft: d: introduce new opensysusers-systemd package

This new package now contains all the parts of opensysusers which step into systemd's namespace.

This way, diversions are no longer needed in opensysusers, and the package becomes trivially co-installable with systemd. As a downside, opensysusers cannot be used anymore as a systemd-sysusers implementation on systems which have systemd installed (i.e. Debian linux-any machines).

The new opensysusers-systemd package now contains the opensysusers-sysusers -> systemd-sysusers symlink, and all the relevant manual pages. It also conflicts with systemd.

See systemd-team/systemd!274 (merged) for some more context.

Gbp-Dch: Full