sdap_save_user: save original primary GID of subdomain users
    
    If ID mapping is enabled we use magic private groups (MPG) for
    subdomains, i.e. the UID and the primary GID of the user will have the
    same numerical value. As a consequence the information about the
    original primary group might get lost because neither in AD domains nor
    on a typical UNIX system the user is an explicit member of it's primary
    group.
    
    With this patch the mapped GID or the original primary group is saved in
    the cached user object under a new attribute.
    
    Fixes https://fedorahosted.org/sssd/ticket/2027