ox: Fix custom link handling in anonymous back-end

ox: Fix custom link handling in anonymous back-end

* lisp/ox.el (org-export-custom-protocol-maybe): Change signature.
* contrib/lisp/ox-groff.el (org-groff-link):
* lisp/ox-ascii.el (org-ascii-link):
* lisp/ox-beamer.el (org-beamer-link):
* lisp/ox-html.el (org-html-link):
* lisp/ox-latex.el (org-latex-link):
* lisp/ox-man.el (org-man-link):
* lisp/ox-md.el (org-md-link):
* lisp/ox-odt.el (org-odt-link):
* lisp/ox-org.el (org-org-link):
* lisp/ox-texinfo.el (org-texinfo-link): Apply signature change.

* testing/lisp/test-ox.el (test-org-export/custom-protocol-maybe):
  Update test.

Provide explicitly back-end used instead of guessing it from INFO
channel as an anonymous back-end could be used, masquerading the real
one.

Reported-by: Christian Moe <mail@christianmoe.com>
<http://permalink.gmane.org/gmane.emacs.orgmode/95402>