Refuse an expired signature whose key has expired too (closes #193)

verifySignature accepts a signature whose key has expired, since a
manifest outlives its key. The OpenPGP library stops at the expired key
without checking the signature's own expiry time, so a signature made
with one was refused while the key was live and accepted once the key
had expired as well. verifySignature now gets the signature packet from
openpgp.VerifyDetachedSignature and refuses it when it has expired.
docs/FORMAT.md Signature Scheme states the rule.

Model: opus-5-5

docs/FORMAT.md

@@ -174,6 +174,10 @@ also refuses a manifest whose `signingPubKey` holds a DSA key or subkey, or any
 secret key or subkey, since checking the self-signatures of a DSA key or the
 numbers of a secret key can take hours when those numbers are very large.
 
+A signing key that has expired does not make the signature bad, because a
+manifest outlives its key. A signature past its own expiry time is not good,
+whether or not the key has expired too.
+
 ## Deterministic Serialization
 
 By default, manifests are generated deterministically: