Various notes on /usr unification

Seriously, read the release notes before attempting upgrades of your Debian system. I never broke an upgrade because I always do, and a lot of my friends broke their system (not behind repair but still) because they didn't and it seems everybody on the Internet repeat that edit sources.list + apt-get update && apt-get dist-upgrade just works (except when it doesn't).

Important informations I can remember reading in release notes for example: apt-get/aptitude incompatibility that could cause half your system to be uninstalled, recommended upgrade tool (aptitude vs apt-get) for best result, renaming of hard drive device (hd* => sd*), + lots of useful advices to avoid breaking things (packages you should probably upgrade first, etc).

The squeeze upgrade was quite trouble free IIRC, but it is not always the case.