bib2html: set TEXMFOUTPUT to the tempdir.

bib2html: set TEXMFOUTPUT to the tempdir.

With the latest update of TeX Live 2009, bibtex became a "safe" command
in that it cannot write output to any directory other than

  (a) a subdirectory of the current directory
  (b) a subdirectory of TEXMFOUTPUT

Setting TEXMFOUTPUT to the temporary directory permits bibtex to write
in that directory.

Additionally, use mkstemp() instead mktemp(), since the latter function
has been deprecated since Python 2.3.