Let's extend nss-systemd to also synthesize user/group entries for the
UIDs/GIDs 0 and 65534 which have special kernel meaning. Given that nss-systemd
is listed in /etc/nsswitch.conf only very late any explicit listing in
/etc/passwd or /etc/group takes precedence.
This functionality is useful in minimal container-like setups that lack
/etc/passwd files (or only have incompletely populated ones).
