manpages-index.xml is just to easily get at each individual man page
with xsltproc. It wasn't a complete docbookx xml file. Now that it is
we can validate it with xmllint. It doesn't fully validate, but we
are close.
This makes the rule for xmllint easier since it doesn't need to
override the DTD to validate against. It also helps with other tools
tryinf to process the docbookx xml files.
