Info backend now takes care to avoid magic characters in node names

Info backend now takes care to avoid magic characters in node names
and index terms (the Info format doesn't like them). In the course
of this I've had to introduce some infrastructure for carrying a
filepos forward from the definition of every RHS index term so that
a particular backend can provide a usefully localised report of
which index term had a problem.

git-svn-id: svn://svn.tartarus.org/sgt/halibut@4051 cda61777-01e9-0310-a592-d414129be87e