athena-auto-update needs a full rewrite

[jdreed]

Given #1020, #1145, and #840, we need to re-write auto-update, because it's now full of hacks. The consensus is that we should switch to apt-get, which has better multi-arch support. But, we need to preserve the check for not removing metapackages. Perhaps this can be accomplished with apt-get -s?

OTOH, if we don't set autoremove (and it's not set by default), apt-get would only remove the actual metapackages, and not their dependencies, which is less catastrophic. So you'd have a functional machine, but machtype -L and possibly the greeter would yell at you.