export predicted-ports via control interface

In #4718 (closed) Mike made Torbutton launch a new request every 12 hours.

This basically torpedos Tor's "go idle and don't build circuits or fetch descriptors when no requests lately" code.

We should export "whether we're dormant" via a getinfo.

I set the milestone to 0.2.2 since I think Mike is expecting his feature to get into a TBB 2.2 release.