Implement `org-export-custom-protocol-maybe' and use it
* lisp/ox.el (org-export-custom-protocol-maybe): New function.
* 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-texinfo.el (org-texinfo-link): Use new function.
* testing/lisp/test-ox.el (test-org-export/custom-protocol-maybe): New
test.