asciidoc: disable network dtd fetching.

Author: acg

doc/default.1.build

@@ -1,5 +1,5 @@
 build_ifchanged "$2.txt"
-safe a2x -L -f manpage "$2.txt"
+safe a2x --format manpage --no-xmllint --xsltproc-opts "--nonet" "$2.txt"
 safe cat "$1"   # a2x can't write to stdout? :/
 exit $?