Rename -tor to -onion to better reflect what it does.

I've seen users confused multiple times thinking they
 should be using -tor to set their tor proxy and then
 finding in horror that they were still connecting to
 the IPv4 internet.

Even Jeff guesses wrong about what the knob does, so
 I think we should rename it. This leaves the old
 knob working, we can pull it out completely in a
 later release.
Merge pull request #2981 from gmaxwell/tor2onion

Rename -tor to -onion to better reflect what it does.