nixos/cups-pdf: init

Some implementation notes:

* cups-pdf, and cups-pdf-to-pdf,
  support multiple instances with differing configurations.
  This can be accomplished by creating multiple configuration
  files with names `cups-pdf-{instance-name}.conf`.
  The Nixos module supports this feature by providing
  the option `instances` which is an attrset
  mapping instance names to instance configurations.
  To simplify module usage,
  an instance `pdf` is created by default.

* To use a cups-pdf instance, one also needs
  a cups queue that connects to the backend.
  The module does this automatically by default,
  using the `hardware.printers.ensurePrinters`.
  It uses one of the ppd files which is
  included in the cups-pdf package.
  If this isn't desired (e.g. because printer queues
  should be created by hand, or configured differently),
  the `installPrinter` option can be turned off
  (for each instance separately).

* In our configuration, cups calls external programs
  using the `cups` account and the `lp` group.
  cups-pdf refuses to operate without root privileges,
  likely because it needs to change the
  ownership of it output pdf files so that
  (only) the print job's owner can access them.

  The module installs a suid root wrapper for the backend
  program that can only be called by the `lp` group.
  The cups-pdf package is replaced by a wrapper
  package which calls the suid root wrapper.
  So cups can call its backend programs as usual.

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