xsltproc --nonet and http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xs mutually incompatible.
Producing the rather unsurprising
I/O error : Attempt to load network entity http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl
Solution: Remove the --nonet flag from both
It might be worthwhile to note that docbook's active development has moved to Github and that the most recent version of that script is
On the other hand since the man pages haven't been touched in the 6 years since they were originally created maybe it would make more sense to just convert them to troff and put the result in the repo, removing the need for xsltproc altogether.