cups/ppd-cache.c: Add cupsUrfSupported to generated PPD (see #804)

cups/ppd-cache.c: Add cupsUrfSupported to generated PPD (see #804)

It fixes driverless printing on Pantum BM5100ADW Series and, probably,
on many other devices.

- Without this parameter, /usr/lib/cups/filter/universal generates RGB
image/urf even for monochrome printer
- Pantum BM5100ADW Series rejects to print these RGB images with
the "Print job canceled at printer" status.
- Probably, this issue affects many other devices

See #804 for details.