DocBook: Generate consistent IDs

By default, DocBook XSL uses a non-deterministic function to generate
IDs for HTML elements where it can't take a name from the input
document.  However, it has the option to generate 'consistent'
(deterministic) IDs instead.  Enable this to make the HTML pages
reproducible.

Reported-by: Jérémy Bobbio <lunar@debian.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>