Major rewrite; use standard session codepath

- Drop the r-kiosk extension because it's hopelessly bitrotted.
- Don't spawn our own VT; go through the standard login session
  codepath (Trac: #628)
- Drop support for things that are not lightdm
- Change the .desktop file to launch the xsession
- Move the kiosk user homedir to /var/lib
- Switch from a modal dialog to notify-send to tell users that they
  don't get to print (Trac: #1424)
- Switch to dh7, and update build dependencies
- Depend on the version of lightdm-config that special-cases the kiosk
  session
- Update the control file to discourage people from using this (since
  it enables nopasswdlogin for a user)