Finally I realized why we were using secondary sorting on site_id

Finally I realized why we were using secondary sorting on site_id
by default :-)  I was originally searching for some magic inside the
search server which needed that to work and completely missed the
simple fact that the front-end wants the results this way :-)

So I'm resurrecting it, but now as an ordinary instance of the secondary
sorting code I've introduced yesterday. The CUSTOM_SORTING switch is gone,
sorting by site ID and page age works always.

Also, I've simplified reverse sorting by introducing a separate XOR mask.