Add properties to footnotes during export
* lisp/org-footnote.el (org-footnote-normalize): add `org-footnote'
property to footnote markers when preparing for exportation.
* lisp/org-html.el (org-export-as-html): read new property to decide
when to export a footnote.
* lisp/org-docbook.el (org-export-as-docbook): read new property to
decide when to export a footnote.
* lisp/org-latex.el (org-export-latex-preprocess): ensure footnote at
column 0 cannot end a list containing it by adding
`original-indentation' property to it.