Debian sources: don't use next release's backports.

Debian sources: don't use next release's backports.

The APT pinning we ship does not support that, and this seems a bit too much of
a corner case to me to deserve being supported out-of-the-box.

Anyone willing to use current release + next release + next release backports
(e.g. Lenny + Squeeze + squeeze-backports) can anyway do so using
apt::sources_list and apt::preferences_snippet.

https://labs.riseup.net/code/issues/2856 is the bug that triggered this change.