Add Upstart job; tweak initscripts

Per (Trac: #1492), there is a weird race condition on Trusty, due
to the avahi-cups-reload Upstart job reloading CUPS, and clashing
with our restarting CUPS in configure-athena-printers.

- 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