Support user:group notation by sysusers.generate-pre.sh script
    
    ... (rhbz#2103934)
    
    u     user_name  uid:gid             "User Description" /home/dir      /path/to/shell
    
    According to: https://www.freedesktop.org/software/systemd/man/sysusers.d.html
    
    (cherry picked from commit 83301531c0aa22ffc155096826d8f5250c6a880a)