user-util: synthesize user records for "nobody" the same way as for "root"

user-util: synthesize user records for "nobody" the same way as for "root"

We already synthesize records for both "root" and "nobody" in
nss-systemd. Let's do the same in our own NSS wrappers that are supposed
to bypass NSS if possible. Previously this was done for "root" only, but
let's clean this up, and do the same for "nobody" too, so that we
synthesize records the same way everywhere, regardless whether in NSS or
internally.