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
Add a helper script debian/extra/make-sysusers-basic which generates a
sysusers.d(5) file from Debian's static master passwd/group files.
systemd 238 now supports specifying different uid and gid and a
non-default login shell, so this is possible now.
Closes: #888126