Switch to XInclude from SYSTEM entities for module inclusion to work around

Switch to XInclude from SYSTEM entities for module inclusion to work around
a bug in xsltproc which prevents xref resolution in complex documents
(http://bugzilla.gnome.org/show_bug.cgi?id=96925).

This pretty much ties us to xsltproc for the time being. To use another
XSLT engine, try running the top-level file through 'xmllint --xinclude'
first.

Running things through 'xmllint --noent' is also an effective work around,
but xinclude is nicer for most of our purposes.

Also corrects a nesting problem in the residue decode section.

svn path=/trunk/vorbis/; revision=4063