A better fix for Upstart issues with cupsys-config

- Ship an upstart job "configure-athena-printers", which starts when
      CUPS is started, and tries up to 5 times to configure the printers.
      We are deliberately NOT using dh_installinit here, since this Upstart
      job can and should exist in parallel with the rc script, so we can
      easily support non-Upstart distros without adding the compatibility
      shims that dh_installinit wants.
    - Tweak configure-athena-printers to take a second argument ("init"),
      which indicates that the script was started from init, and thus it
      should expect CUPS to be running, and not try to restart it.
    - Tweak the initscript to be aware of the upstart job, and to not do
      anything if it looks like Upstart will deal.
    - Add a note for future generations in restart_cups.sh, should we
      ever decide to use that again.
    - Tell dh_installinit to start the initscript as late as possible (this won't affect upgrades, which is probably fine)