Adjust the rbc downwards after a buffer-flush, to simplify debugger interrupts.

Resolves #183.  Resolves #201.

As an aside, this was a relatively simple bug that was tremendously difficult to find because of bad luck; I kept seeing rcb values like, `start_rbc < failure_rbc < target_rbc`, so it looked like execution was advancing normally but failing during the continue.  And the code that happened to be failing in FF was some crazy xpcom/JS-module junk that made me wonder if we'd missed some source of nondeterminism.  Need to be more arrogant in the future! :)