Some distros install nologin as /usr/sbin/nologin, others as /sbin/nologin. Since we can't really on merged-usr everywhere (where the path wouldn't matter), make the path build time configurable via -Dnologin-path=. Closes #13028
Do not merge group creation with user creation because with the new uid:gid syntax this can result in confusing (and unwanted) behavior.