tree: port all user/group creation scripts to systemd-tmpfiles and systemd-sysusers

[MingcongBai]

Given that there is virtually no environment constraints where systemd-tmpfiles and systemd-sysusers could be run (although tmpfiles fails in chroot environments, should be a bug, will report), I propose to port:

• autobuild/usergroup files.
• getent-or-create postinst files that creates user(s) and/or group(s).

To systemd configuration files for creating system users and directories.

See: https://www.freedesktop.org/software/systemd/man/systemd-sysusers.html, and https://www.freedesktop.org/software/systemd/man/systemd-tmpfiles.html.

[MingcongBai]

Upstream issue: https://github.com/systemd/systemd/issues/4954.