Default to changelogs being always online for Debian

Per discussion on debian-devel, debhelper will as of next upload automatically trim Debian changelogs for very old entries.

As I understand it, this means we should strongly consider making AlwaysOnline the default in apt for Debian as well. Proposed is a minimal patch for that based on how it was done for Ubuntu (but I am unsure if it is the best approach for Debian as well)