util: add sha256 Upcoming patches will teach host fingerprint checking to verify non-SHA1 fingerprints. Many x509 certificates these days are SHA-256. And modern browsers often display the SHA-256 fingerprint for certificates. Since SHA-256 fingerprints are highly visible and easy to obtain, we want to support them for fingerprint pinning. So add SHA-256 support to util. I did not add SHA-256 to DIGESTS and DIGESTS_BY_STRENGTH because this will advertise the algorithm on the wire protocol. I wasn't sure if that would be appropriate. I'm playing it safe by leaving it out for now.