Improve subsidiary items for exercises (hints, etc)

[rbeezer]

Hints, answers, solutions presently are always born as knowls in the HTML version. The switches like exercise.backmatter.hint only control actual accessibility (see top of xsl/mathbook-common.xsl). I expect them to change from yes/no to something like visible/knowl/invisible with a defined interpretation of knowl when knowls are not possible (eg print/PDF). I think there is some inconsistency with hints, etc since they can be rendered twice (in text, in backmatter solutions list), especially as targets of cross-references. I might only allow them to be rendered at most once (text or backmatter).

Check treatment of inline versus sectional, especially in solutions list.