HBCK improvements

We just had a prod issue and running HBCK the way we did actually causes more problems.
In part HBCK did stuff we did not expect, in part we had little visibility into what HBCK was doing, and in part the logging was confusing.

I'm proposing 2 improvements:
1. A dry-run mode. Run, and just list what would have been done.
2. An interactive mode. Run, and for each action request Y/N user input. So that a user can opt-out of stuff.

jmhsieh, FYI