gpg: Rework the signature subpacket iteration function.

* g10/parse-packet.c (enum_sig_subpkt): Replace first arg by two args
so that the entire signature packet is available.  Change all callers.
(parse_sig_subpkt): Ditto.

This patch is a prerequisite to support the new attestation key
signatures.

• Signed-off-by: Werner Koch <wk@gnupg.org>