Improve footnote handling

- Footnotes separator rather than headline to get around i18n
- Warn on footnote redefinition
- Do not export footnote definitions at point of definition, only in the
  footnote section.
- Do not automatically exclude Footnotes section to get around possibly hiding
  other content of such a section - and i18n.
  The user has the choice of explicitly hiding the section via a :noexport:
  tag.

and some other refactoring

org/testdata/misc.html

@@ -359,8 +359,11 @@ When inserting an image link like <img src="/home/amos/Pictures/Screenshots/img-
   (ansi-term))
 </pre>
 </div>
+<h1>
+Footnotes
+</h1>
 <div class="footnotes">
-<h1 class="footnotes-title">Footnotes</h1>
+<hr class="footnotes-separatator">
 <div class="footnote-definitions">
 <div class="footnote-definition">
 <sup id="footnote-1"><a href="#footnote-reference-1">1</a></sup>