Adding setup-hook for opensp, to set the sgml catalog files

This sets the SGML_CATALOG_FILES, similar to how libxml2 does the same with
XML_CATALOG_FILES.

I based the hook on the libxml2 one, and I followed the instructions about this
variable here:
http://docbook.sourceforge.net/release/dsssl/1.79/doc/install.html

pkgs/tools/text/sgml/opensp/setup-hook.sh

@@ -0,0 +1,22 @@
+addSGMLCatalogs () {
+      if test -d $1/sgml/dtd; then
+          for i in $(find $1/sgml/dtd -name docbook.cat); do
+              export SGML_CATALOG_FILES="${SGML_CATALOG_FILES:+:}$i"
+          done
+      fi
+}
+
+if test -z "$sgmlHookDone"; then
+    sgmlHookDone=1
+
+    # Set http_proxy and ftp_proxy to a invalid host to prevent
+    # xmllint and xsltproc from trying to download DTDs from the
+    # network even when --nonet is not given.  That would be impure.
+    # (Note that .invalid is a reserved domain guaranteed not to
+    # work.)
+    export http_proxy=http://nodtd.invalid/
+    export ftp_proxy=http://nodtd.invalid/
+
+    export SGML_CATALOG_FILES
+    envHooks=(${envHooks[@]} addSGMLCatalogs)
+fi