Fix bug in HTML index generation: any index tags appearing in a

Fix bug in HTML index generation: any index tags appearing in a
section title (which is not remotely unreasonable) got index
references added to them not only when the section title was
processed as part of its section, but also when it was processed as
part of contents navigation displays - in which situation it tended
to cause a link to, for example, the last section in the _previous_
file. Now we keep constant track of whether or not we're supposed to
be indexing things as we process text.

git-svn-id: svn://svn.tartarus.org/sgt/halibut@3991 cda61777-01e9-0310-a592-d414129be87e