Add support for Debian Squeeze LTS updates.

For more information please read:
http://www.debian.org/News/2014/20140616
http://www.debian.org/News/2014/20140424
Use dynamic mirror selection provided by Debian instead of forced static.
Use _MIRROR variable if possible.
Add support for Debian Squeeze Stable Proposed Updates.
Latest hotfix added on: Wed Jul  9 09:00:32 CDT 2014