Fix wrong "nobody" group from sysusers.d

Fix our make-sysusers-basic sysusers.d generator to special-case the
nobody group. "nobody" user and "nogroup" group both have the same ID
65534, which is the only special case for Debian's static users/groups.
So specify the gid explicitly, to avoid systemd-sysusers creating a
dynamic system group for "nobody".

Also clean up the group on upgrades.

Thanks to Keh-Ming Luoh for the original patch!

Closes: #912525