ulink.footnotes
— Generate footnotes for ulinks?
If non-zero, and if ulink.show also is non-zero,
the URL of each ulink will appear as a footnote.
DocBook 5 does not have an ulink element. When processing
DocBoook 5 documents, ulink.footnotes applies to all inline
elements that are marked up with xlink:href attributes
that point to external resources.