De-kludgify marking circs as unsuitable for new streams

      /* XXXX024 This is a screwed-up way to say "This is too dirty
       * for new circuits. */
      circ->timestamp_dirty -= options->MaxCircuitDirtiness;

    /* XXXX024 this is a kludgy way to do this. */
    tor_assert(circ->timestamp_dirty);
    circ->timestamp_dirty -= options->MaxCircuitDirtiness;

    /* XXXX024 this is a kludgy way to do this. */
    tor_assert(circ->_base.timestamp_dirty);
    circ->_base.timestamp_dirty -= get_options()->MaxCircuitDirtiness;

Yes, it is, and wrong in several different ways too.