Improve tooling and usability for approved-routers file and its allies

I gather from the directory authority operators that the current approved-routers situation is a royal pain. Mistakes can cause authorities to exit; address ranges need to go in one place while fingerprints go in another; different flags are forced on and off in different ways, there is no 'lint' tool, and nothing is fun.

We should design an improved interface for this, lest we prevail too much upon their patience.