gpg: Silence some diagnostics.

* g10/parse-packet.c (enum_sig_subpkt): Show "buffer shorter than
subpacket" only in debug mode.
(parse_signature): Show "signature packet without timestamp / keyid"
only in souble verbose mode.
* g10/sig-check.c (check_signature_metadata_validity): Use ISO
timestamp in UTC for the signature expired note.

I have seen to many of these diagnostics and in particular the first
one seems to be a connected to the others. Thus it does not make
sense to show them in standard verbose mode.

The ISO timestamp is much easier to read than than the localized
timestamp and switching from localtime to UTC should not harm.