bump the libxml2 require to 2.6.0 which should ship for good real soon...

* configure.in: bump the libxml2 require to 2.6.0 which
  should ship for good real soon...
* doc/Makefile.am: fix installation of HTML pages
* doc/libxslt-api.xml: rebuilt
* libxslt/xsltexports.h: cleanup
* python/generator.py: ATTRIBUTE_UNUSED is after the parameter
* xsltproc/xsltproc.c: applied Crutcher Dunnavant --load-trace patch 2
Daniel