Allow revoking an entire signing key

[nsheridan]

The sshd option TrustedUserCAKeys allows the specified CA file to contain multiple keys which makes rotating signing keys pretty easy. But if the signing key is compromised we should be able to revoke all certs by revoking the signing key - the sshd will automatically deny any cert signed with the revoked signing key.
This sounds easy enough to implement, but we don't currently record any data about the signing key and we probably should.

@patrickod I'm interested in your thoughts on this