Install and use sysusers.d config files

sysusers.d config files allows a package to use declarative configuration instead of manually written maintainer scripts. Install files for the packages using adduser, and switch over. 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

Also stop deleting the user on purge, as that is considered bad practice, as any potential leftover file/directory can then become owned by the next user/group that gets added, with unpredictable consequences.