Install and use sysusers.d/tmpfiles.d config files

Use systemd-sysusers and systemd-tmpfiles for system user/group creation and state file ownership, instead of manual adduser/chown invocations in the postinst.

sysusers.d and tmpfiles.d config files allow a package to use declarative configuration instead of manually written maintainer scripts. This also allows image-based systems to be created with /usr/ only, and also allows for factory resetting a system and recreating /etc/ on boot.

https://www.freedesktop.org/software/systemd/man/latest/sysusers.d.html https://www.freedesktop.org/software/systemd/man/latest/tmpfiles.d.html

Note that this does not require a hard dependency on systemd, debhelper generates depetencies correctly so that they work out of the box on non-systemd and non-linux Debian builds seamlessly.