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