Introduced org-lparse-begin-collect and org-lparse-end-collect

Introduced org-lparse-begin-collect and org-lparse-end-collect

* contrib/lisp/org-lparse.el (org-lparse-footnote-buffer):
Deleted.  Genericized as `org-lparse-collect-buffer'.
(org-lparse-output-buffer): Added docstring.
(org-lparse-collect-buffer): New variable.  Was
`org-lparse-footnote-buffer' till now.
(org-lparse-collect-count): New variable to aid in asserting
implicit assumptions made by the export engine.
(org-lparse-begin-collect): New function.  Factored out from
`org-lparse-begin-footnote-definition'.
(org-lparse-end-collect): New function.  Factored out from
`org-lparse-end-footnote-definition'.
(org-lparse-begin-footnote-definition)
(org-lparse-end-footnote-definition): Use above functions.
(org-do-lparse): Reset `org-lparse-collect-buffer' and
`org-lparse-collect-count' at the beginning of export.
Cleanup `org-lparse-collect-buffer'.

* contrib/lisp/org-xhtml.el (org-xhtml-end-export): Removed
references to `org-lparse-footnote-buffer'.