Rename HSDir consensus flag to OnionDir

Over time, nobody is going to know what an HSDir is. Eventually we will want to use a more understandable name. I propose OnionDir.

Step 1: Make clients able to parse the OnionDir flag, and they just treat it as a synonym for HSDir.

Step 2: Wait a long time until all clients from before step 1 are gone.

Step 3: Have dir auths start saying OnionDir instead of HSDir.

(In the above plan, step 2 could take years. But I think it's still the simplest plan. The only real risk is if we somehow screw up with step 1, and don't notice the screw-up until we try step 3.)