Install and use sysusers.d config file

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 tcpdump user/group on purge. This is widely considered an anti-pattern, as any potential leftover file/directory can then become owned by the next user/group that gets added, with unpredictable consequences. There's no way to know for sure that all files owned by the deleted user/group have been removed from all filesystems of a system. A valid uid might be used to own any file on any filesystem on a machine, and there's nothing realistic a package can do to find them all, as an in-depth scan of all filesystems is not really feasible from a maintainer script.