Added XML Catalog resolution of catalog.dtd. This involved:
* Creating a new command "join-strings" for creating Java classpaths.
* Creating a new command "saxon", which encapsulates mechanics of making saxon work, instead of a direct java call.
* Fetching the catalog.dtd from the network, and installing it in pkg/.
* Fetching the XML Catalog entity resolver from the network, and installing it in pkg/
* Creating a catalog that captures the DTD reference.
* Adding the ShA1 hash for the DTD.
* Adding fetch & install code to tehe Makefile
Added hash for additional package