Clarifies intention, and with IterCount::NONE prepared to handle
something like old PBKDF1, if anything actually used that. See
https://tools.ietf.org/html/rfc8018#section-5.1 where iteration
count is not part of the re-hash.

Change-Id: I5f97ca7a91f611eced8ced0a0c64961c04535d36