Enforce that \q may not be used anywhere within \c. It shouldn't be

Enforce that \q may not be used anywhere within \c. It shouldn't be
necessary since the whole point of \c should be that the user wants
to exercise exact control over the glyphs used, and forbidding it
has the useful effect of relieving some backends of having to make
difficult decisions: it means the text backend doesn't have to nest
two pairs of identical quotes, and the paper backends don't have to
downgrade their quote characters if (as is perfectly plausible) the
fixed-pitch font doesn't support the same range as the body text
fonts.

git-svn-id: svn://svn.tartarus.org/sgt/halibut@4277 cda61777-01e9-0310-a592-d414129be87e