Initial checkin of the shiny new rewritten-from-scratch HTML back

Initial checkin of the shiny new rewritten-from-scratch HTML back
end. There's a lot more _potentiality_ for new features than there
are actual new features just yet, but future highlights include:
configurable flavour of HTML (3.2, 4, XHTML Transitional or Strict),
proper character set support (this is half way there already), and
more flexible allocation of sections between multiple HTML files.
Meanwhile, immediate benefits include correct handling of special
characters within `author' and `description' strings, omission of
the filename part in hyperlinks within the same HTML file (in
particular, this means a single output file is now totally
independent of its filename), and hyperlinks to the index from the
top-level contents page (I'm amazed nobody has complained at the
lack of this yet!). There are no doubt some shiny new bugs as well,
but I'll never find them unless people start using the thing...

git-svn-id: svn://svn.tartarus.org/sgt/halibut@4275 cda61777-01e9-0310-a592-d414129be87e