enforce sane readdir usage
    
    Now that all clients have been adjusted, ensure that no future
    misuse of readdir is introduced into the code base.
    
    * cfg.mk (sc_prohibit_readdir): New rule.
    * src/util/virfile.c (virDirRead): Exempt the wrapper.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>