signature checking changes

Adjust the gpgv signature check. Add one more case of broken key to detect,
EXPKEYSIG. From gnupg docs:
    EXPKEYSIG   <long keyid>  <username>
    The signature with the keyid is good, but the signature was
    made by an expired key. The username is the primary one
    encoded in UTF-8 and %XX escaped.

Also, handle KEYEXPIRED right. The first argument given back is NOT the keyid,
it is the timestamp when the key expired. From gnupg docs:
    KEYEXPIRED <expire-timestamp>
    The key has expired.  expire-timestamp is the expiration time
    in seconds after the epoch.

    Note, that TIMESTAMP may either be a number with seconds since
    epoch or an ISO 8601 string which can be detected by the
    presence of the letter 'T' inside.
So lets go and see if we find a T, if not convert the epoch to something
more easily human readable in our reject message.

Signed-off-by: Joerg Jaspert <joerg@debian.org>