(internal) Added support for TDG hyperlinking with <tag> (instead

(internal) Added support for TDG hyperlinking with <tag> (instead
of just <sgmltag>. Also added support for ignoring <tag>s and
<sgmltag>s that have a "namespace" attribute whose value contains
"http://docbook.sf.net/" or "http://docbook.sourceforge.net/"
(that is, those don't get turned into TDG hyperlinks).

(Note: This change has no effect on the behavior of the DocBook XSL
stylesheets; it only affects the build used to create the actual)
DocBook XSL releases.)