linksexp -- Texinfo/XBEL/OPML/urls autogeneration from recfile bookmark.

Given recfile (https://www.gnu.org/software/recutils/) bookmark file,
this program can generate (export links):

* either part of Texinfo source code output
* or XBEL (http://xbel.sourceforge.net/, https://en.wikipedia.org/wiki/XBEL)
* or OPML (http://dev.opml.org/, https://en.wikipedia.org/wiki/OPML)
* or "urls" file for newsboat (https://newsboat.org/)