willu's teximg changes

* teximg: The prefix is configurable, and has changed to not include the
  nonstandard mhchem by default. (willu)
* teximg: dvipng is used if available to render images. Its output is
  antialiased and better than dvips. If not available, the old dvips+convert
  chain will be used. (willu)
* Drop suggests on texlive-science, add suggests on dvipng.