fill_initgr: add original primary GID if available
    
    In some cases when MPG domains are used the information about the
    original primary group of a user cannot be determined by looking at
    the explicit group memberships. In those cases the GID related to the
    original primary group is stored in a special attribute of the user
    object.
    
    This patch adds the GID of the original primary group when available and
    needed.
    
    Fixes https://fedorahosted.org/sssd/ticket/2027