libxml: Improve copyright headers.
The copyright headers should be unambiguous even if a single file is copied from
the GNU gettext package and used elsewhere. Reported by John Darrington.
* gnulib-local/lib/libxml/*: Inline this directory's COPYING file where
applicable. Add FSF copyright header.
* gnulib-local/lib/libxml/trionan.c: Add copyright header.
