muse-latex2png: Parametrize and create subdirectory when needed.

muse-latex2png: Parametrize and create subdirectory when needed.

* lisp/muse-latex2png.el Rename all functions to have a "muse-latex2png"
  prefix.  Turn all variables into customizable options in the
  muse-latex2png group.
  (muse-latex2png-img-dest): New option specifying where to place the
  images.
  (muse-latex2png-template): New option containing the template to use
  for the surrounding LaTeX code.
  (muse-latex2png-use-xhtml): New option that toggles strict XHTML
  compliance.
  (muse-latex2png-move2pubdir): Avoid multiple redundant error messages
  when something else goes wrong.  Create the image subdirectory if it
  doesn't exist already -- thanks to Christian Straßer for the report.
  Expand the filename properly.
  (muse-publish-latex-tag, muse-latex2png): Ditto on error messages and
  filename expansion.
  (muse-latex2png): Use the "muse-latex2png" prefix by default.  Set the
  default directory properly.
git-archimport-id: mwolson@gnu.org--2006/muse--main--1.0--patch-183