CONFDB/SYSDB: Add the hybrid MPG mode
    
    Permits a new option value 'hybrid' for the auto_private_groups option.
    The option was even previously marked as a string option in both the
    configAPI and the man pages, so we don't have to change the type now.
    
    If the hybrid mode is selected and the user's original GID number is
    available, then during initgroups and getpwnam, it is used as their primary
    GID instead of the MPG group. The original group is also not added
    as a secondary group during initgroups in this case.
    
    Related:
    https://pagure.io/SSSD/sssd/issue/3822
    
    Reviewed-by: Michal Židek <mzidek@redhat.com>