Work around Xerox driver resource leak

The Xerox drivers appear to have a resource leak
that leaks about 1MB and 4 handles in the COM+
hosting process for every installed Xerox printer
when PrintServer.GetPrintQueues() is called.

Use the EnumPrinters() Winspool.drv function
to query the list of shared local printers, and
cache the PrintQueue objects returned from
PrintServer.GetPrintQueue(name)