Add the check flag for decryption validation

[enrichman]

At the moment is not possible to know how many parts are needed to decrypt the secret. It could be possible adding a couple of informational bytes to the partial keys but I'm not sure if this could be a security issue. I would ask to a security expert about it.

Anyway since we have the checksum of the unencrypted secret we could add a --check flag to validate that the decryption was successful or not.