Add helper to check if a signature is good
In some situations we want to know whether a data signature is good, i.e.
it matches the signed data, even if the signing key has expired or has
been revoked or the signature itself has expired. In this case the new
signatureIsGood can be used. It's complemented by signatureIsBad.
Note that checking if a signature is good is not sufficient to decide
whether the signature is trust worthy.
(cherry picked from commit 3de2e13e0e969d780348a09158b90e94dc5d621f)