Header list restructuring, details

[mnot]

More fine-grained detail would be nice in the list of headers.

Specifically:

1. colour-code according to whether we know about the header or don't, and whether a problem is detected or not. The state of parsed headers can be used for much of this.
2. Separate checks into per-line vs. whole set, so that we can target the specific header that's a problem, rather than all instances of it. For example, if one cache-control directive has a problem, but cc is split among many lines, we should be able to only highlight the specific line (eventually, we'll get to directive level, but not in this ticket).