gnulib-local: Update included libxml2 to 2.9.3
* gnulib-local/lib/libxml/*: Update from libxml2 2.9.3. Add buf.[ch],
enc.h, save.h, triodef.h, trionan.[ch]. Enable XPath and XPointer
support.
* gnulib-local/modules/libxml (Files): Add new files from libxml2 2.9.3.
