RESPONDER: Add a helper function sss_resp_create_fqname
    
    When looking up entries in the responders that have not been yet
    converted to the cache_req API, we need to perform some common
    operations all the time. These include converting the name to the right
    case, reverse-replacing whitespace and converting the name to the qualified
    format for that domain.
    
    This patch adds a function that performs these steps to avoid code
    duplication.
    
    Reviewed-by: Sumit Bose <sbose@redhat.com>