allow multi-session between tor and non-tor

[slingamn]

I disallowed this:

https://github.com/oragono/oragono/blob/1b0e9a0c081852e435a1b760c8b47103c3b586fd/irc/client_lookup_set.go#L158

but I don't think my reasons for doing so make sense. If we disallow the reattach, the client gets a 433 ERR_NICKNAMEINUSE and responds by adding an underscore to the nick or whatever. But they still get to log in and they still get their account-tag and associated privileges, like channel amodes --- so the server is publicly confirming their identity and associating it with their real IP address.

I think this should be allowed, and instead we should have a user preference to disallow access to the account over non-Tor connections.