SELINUX: Use getseuserbyname to get IPA seuser
    
    The libselinux function getseuserbyname is more reliable method to retrieve
    SELinux usernames then functions from libsemanage `semanage_user_query`
    and is recommended by libsemanage developers.
    Replace get_seuser function with getseuserbyname.
    
    Resolves:
    https://pagure.io/SSSD/sssd/issue/3308
    
    Reviewed-by: Michal Židek <mzidek@redhat.com>
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    Reviewed-by: Petr Lautrbach <plautrba@redhat.com>