AD/LDAP: do not fall back to mpg user lookup on GC connection
    
    For MPG domains a group lookup might fall back to a user lookup to check
    if the request is for a user private group. Since we cannot be sure that
    all needed attributes for a user are replicated to the Global Catalog we
    do not want to lookup the user during the fall back from the Global
    Catalog.
    
    Since we cannot skip Global Catalog lookups for groups completely due to
    membership to groups with universal scope this patch adds a flag to tell
    the lower level lookup calls to not fall back on connections to a Global
    Catalog.
    
    Related to https://pagure.io/SSSD/sssd/issue/3748
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>