Merge branch 'bc/use-asciidoctor-opt'

Merge branch 'bc/use-asciidoctor-opt'

Asciidoctor, an alternative reimplementation of AsciiDoc, still
needs some changes to work with documents meant to be formatted
with AsciiDoc.  "make USE_ASCIIDOCTOR=YesPlease" to use it out of
the box to document our pages is getting closer to reality.

* bc/use-asciidoctor-opt:
  Documentation: implement linkgit macro for Asciidoctor
  Makefile: add a knob to enable the use of Asciidoctor
  Documentation: move dblatex arguments into variable
  Documentation: add XSLT to fix DocBook for Texinfo
  Documentation: sort sources for gitman.texi
  Documentation: remove unneeded argument in cat-texi.perl
  Documentation: modernize cat-texi.perl
  Documentation: fix warning in cat-texi.perl