Rip out CUPS code; fix policy-rc.d; tweak UI

- Rip out the last of our browsepolling code, and the complicated
  policy-rc.d that it required.  (I don't care how bad printing
  is, we are never going back to BrowsePolling.)
- Don't install policy-rc.d in /usr/sbin because d-i is
  incompetent (Trac: #1478)
- Inform the user when we break their login session (as opposed to when
  they break their login session)
Update for Trusty and schroot 1.6

schroot 1.6 introduces some configuration changes and deprecates
script-config.  It still works, but spews to stderr, so let's make
it go away:
- In the 'login' chroot config, add a new "profile" key pointing
  at the to-be-installed 'athena' directory which contains the profile.
- Drop the template fstab (mount-athena.in), which was only needed because
  we were still supporting pre-/run releases.
- Drop the corresponding shell code from script-athena, and instead point
  at the new 'fstab' file shipped with the profile mentioned above.
- The new 'fstab' file mounts /run with rbind, because /run/user is a thing
  and it's easier to just do it all at once.
- In the rules file, if we're using an old schroot release, edit the
  config for the old syntax (we still ship script-athena)

Tweak the setup.d scripts for schroot 1.6
- Only source CHROOT_SCRIPT_CONFIG if common-config doesn't exist.
- Update 16killprocs-no-really to use CHROOT_PATH when killing

Tweak the transformation of the defaults file
- schroot 1.6 changes the values.  Also, add a "die" clause so
  we notice when this fails in the future.

Add a .maintscript file to remove the mount-athena.in conffile
Add a misc:Pre-Depends for the .maintscript file.
Bump the Dependency to schroot 1.4.25 (precise) or higher