Fix assumption that a signature on a public key is a revocation

Fix assumption that a signature on a public key is a revocation
We've been assuming that a signature directly on a public key means the
key is revoked. This isn't the case; it's used for key preferences for
example. Change the structure element names to reflect this
(revocations->sigs) and check for revoked status by looking for a
signature type of 0x20.