Changes to user management

This MR changes user management in two ways:

• replaces our use of dh-sysuser with custom {add,del}user calls in post{inst,rm};
• limits adding the 'libvirtdbus' user to the 'libvirt' group to Ubuntu only.

The former is desirable because of #1000478, and ensures we clean up properly on package purge. We can decide to switch back to dh-sysuser at a later date once the issues with it are solved, or just stick with our custom logic which is not that complicated anyway.

The latter is a consequence of me changing my mind compared to !8 (comment 281133) O:-) After thinking about this a bit more, I would now prefer libvirt-dbus to not deviate from the upstream behavior where possible, which means using policykit instead of group membership for permissions. And since having the same permissions configured via two separate mechanisms at once would be confusing, we should keep things as they were before, at least in Debian.