Using dh-sysuser when available

[arvatoth-vcc]

https://manpages.debian.org/stretch/dh-sysuser/dh_sysuser.1.en.html

This debhelper manages the users and groups. Available from the Ubuntu Bionic (18.04) version, and Debian Stretch. You can keep the postinst/postrm scripts it clean.

[heartsucker]

This is a good suggestion. If this is something you would like to add by doing something the following, I would merge it in.

if hash dh_sysuser > /dev/null; then
    # do new stuff
else
    # do old stuf
fi