docbook5-xsl => docbook-xsl-ns, etc.

docbook5-xsl => docbook-xsl-ns, etc.
To ease hassles for downstream packagers, we are going to be
better of naming the namespaced stylesheets "docbook-xsl-ns"
instead of tying them to a specific version of DocBook (i.e.,
DocBook 5). Anyway, the real distinction is in fact that they are
namespaced, not they they are for DocBook 5.