The `nobody` user/group is special and can't be driven from a sysusers
dropin because Fedora's systemd has a compiled-in default value
for naming the overflow user that same name and that always takes
precedence.
The problem is that due to legacy and cargo-culting, we have to deal
with a bunch of systems with the `nobody` user set to 99:99 that we
can't just ignore. We need to migrate those, but for now at least to
make `--sysusers` usable in these environments, let's add a new hidden
`--nobody-99` option which defines _only_ that entry in the hardcoded
passwd/group. This _is_ respected by systemd-sysusers.
See also: https://github.com/coreos/fedora-coreos-tracker/issues/1201
See also: https://github.com/systemd/systemd/issues/7717
4eb52e5483