Bug#809669: unattended-upgrades: files got created under /var/ mountpoint

[Alexandre Detiste]

control: severity -1 important

Let's tag this important: all packages updated in "at shutdown mode"
will have their new /var files stuffed on the wrong hard disk
and these file will be invisible after a reboot;
and package will then consist of a mix of new assets in /
+ old assets in /var.