final_check enforces msg to sign

[grarco]

Currently, final_check prepends the public key for signature verification to the sighash to construct the actual msg on which the signature should be verified. This is the correct logic for the single verifier but the batch one doesn't need this because it will reconstruct the correct message on its own. This lead to a signature verification error when using the batch verifier. We should change final_check to expect the correct msg and let the caller pass the correct value to support both verifications.