interrupted package upgrade should continue on next boot in case of a problem

While I was running an update (`aptitude dist-upgrade`), the power somehow got lost. When turning it on again, the nvidia driver had a version mismatch, and /dev/disk/by-uuid wasn't present, so my home couldn't be mounted. Bulletproof X was started, but didn't work since udev hadn't been started properly (I think). I knew what to do, but an ordinary user would be stuck here.

I propose to (the possibility to) check at boot if dpkg was interrupted, and run `dpkg --configure -a` if it was. I'd say this would be a nice addition to bulletproof X to make it a more general system recovery tool.