import: properly verify roothash_signature + verity download, too

import: properly verify roothash_signature + verity download, too

Follow-up for 133b34f69a72dc90d4e336837d699245390c9f50 where this was
forgotten.

While we are at it, bring the parameters into the same order as we
declare them in the PullRaw/PullTar objects, i.e. match them to the
canonical order.