Use `org-export-current-backend' whenever possible.

Use `org-export-current-backend' whenever possible.

* org-special-blocks.el
(org-special-blocks-make-special-cookies): Use
`org-export-current-backend'.

* org-publish.el (org-publish-aux-preprocess): Use
`org-export-current-backend'.

* org-inlinetask.el (org-inlinetask-export-handler): Use
`org-export-current-backend'.

* org-exp.el (org-export-current-backend): New variable.
(org-export-preprocess-string)
(org-export-format-drawer-function)
(org-export-remove-or-extract-drawers)
(org-export-format-drawer)
(org-export-convert-protected-spaces)
(org-export-select-backend-specific-text)
(org-export-mark-list-end, org-export-mark-list-properties)
(org-export-attach-captions-and-attributes)
(org-export-replace-src-segments-and-examples)
(org-export-format-source-code-or-example)
(org-export-number-lines): Use the new global variable instead
of a local variable.

* org-exp-blocks.el (org-export-blocks-format-ditaa)
(org-export-blocks-format-dot)
(org-export-blocks-format-comment): Use
`org-export-current-backend'.