dissect: wrap verity settings in new VeritySettings structure

dissect: wrap verity settings in new VeritySettings structure

Just some refactoring: let's place the various verity related parameters
in a common structure, and pass that around instead of the individual
parameters.

Also, let's load the PKCS#7 signature data when finding metadata
right-away, instead of delaying this until we need it. In all cases we
call this there's not much time difference between the metdata finding
and the loading, hence this simplifies things and makes sure root hash
data and its signature is now always acquired together.