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)