auto-update should deal with conffile conflicts

[broder]

Right now, if anything triggers a dpkg conffile conflict, auto-update will hang indefinitely. Not the best failure mode.

On cluster machines, since we are the sysadmin, we should be passing -o Dpkg::Options::=--force-confnew to aptitude.

On not-cluster machines, it's less clear. It sounds like Athena 9 used rpm's default behavior, which was the equivalent of --force-confold. I think ideally the update should fail and e-mail root (yeah, yeah, #83, I know), leaving them to fix the upgrade, but I don't know if there's a way to make dpkg error out on conffile conflicts.