doc: Use a version of texi2html which is under a good license.

doc: Use a version of texi2html which is under a good license.

Reported by <ineiev@gnu.org> in <https://savannah.gnu.org/bugs/?54809>.

* gettext-tools/doc/texi2html: Copied from texi2html-1.78, with changes made in
libunistring/build-aux/texi2html and then in libtextstyle/build-aux/texi2html.
* gettext-tools/doc/gettext.texi: Enable the Top node during texi2html
processing.
* gettext-tools/doc/Makefile.am (gettext.html): Use texi2html options
-no-sec-nav -no-menu -toc-links.
(gettext_toc.html): Likewise. Pull out generated files out of subdirectory.