don't purge directly, but remove and do purge at the end

If we want a package to be purged from the system tell dpkg in the
ordering (if it has to touch it explicitly) to remove it and cover the
purging of the config files at the end with a --purge --pending call.

That should help packages move conffiles around between packages
correctly even if the user is purging packages directly in big actions
like dist-upgrades involving many packages.