expand getinfo circuit-status to show build_state flags

We've got a usability bug where users are misinterpreting the circuit listing on Vidalia's network map. In #2114 (closed) I suggest that Vidalia should start giving the user more information.

But Tor really doesn't export enough information for Vidalia to know what to say. Right now we export a PURPOSE= argument, but the string we send isn't very useful. Also, we don't export some flags, like onehup_tunnel, which Vidalia needs to know to tell the user that the circuit in question is probably for fetching directory information.

We should come up with a spec change and implement it. Shouldn't be too hard.