experimental/cups-1.6-and-1.7

opt-out cups update:

The new cupsd-1.6.nix file is default, you can still uncomment the old one.
Both module have a new option "cupsPackages" which is the way to specifiy which
cups/ghostscript versions to use.

1.6.4+ comes with some changes:
- default filter chain is PDF, gs is only used for gs printers
- some stuff has been moved into cupsFilters and is maintained by
  openprinting now
- cups has 2 configuration files now
- some more less important changes such as compressing print files sent over
  the network and whatnot (I didn't test them)

Issues:
- printing test pages does not work yet

fixes:
- printing on canon-bjc like printers using gutentprint drivers.
  1.5.4 only prints 30% of page for whatever reason.

new packages: ijs, qpdf
ghostscript: get back the update which got reverted while keeping the old version

This includes b96fc98306a pull/1115