Fix up the restrict_charset and output_charset configuration for the

Fix up the restrict_charset and output_charset configuration for the
HTML backend. After an argument with James in which it transpired
that we each thought it was absolutely obvious which one
\cfg{html-charset} ought to map to, and each of us was thinking of a
different one, I've decided that this implies it _isn't_ obvious,
and hence there is no single \cfg{html-charset} directive at all.
Instead, we have \cfg{html-restrict-charset} and \cfg{html-output-
charset}, and any naive user who `just wants to change character
set' is just going to have to RTFM and figure out which one they
mean.

git-svn-id: svn://svn.tartarus.org/sgt/halibut@4313 cda61777-01e9-0310-a592-d414129be87e