When we are really satisfied that it is right, tell protover.c about prop224 HSDir support

This will be trivial to do, but nontrivial to know when we should do it.

prop224-supporting hidden services and relays will need to know which Tor relays they should use to upload and download the new kind of descriptors. So, they should use the preferred mechanism for it.

As of proposal 264, that's protocol subversions.

We should only advertise support for this server-side once we're pretty sure we aren't changing the server side any more.