Change how prepend_user_name interacts with args
    
    Change how the prepend_user_name attribute interacts with arguments that
    are to be passed to the helper - the calling user's name is still
    prepended to the list of arguments received in the D-Bus request, but
    now arguments included in the helper configuration are prepended first,
    so the order is now:
      ["helper" binary] ["helper" command line args] [user name] [args from request]
    
    Signed-off-by: Nalin Dahyabhai <nalin@redhat.com>