gpg: Silence some warning messages during -Kv.

* g10/options.h (glo_ctrl): Add flag silence_parse_warnings.
* g10/keylist.c (list_all): Set that during secret key listsings.
* g10/parse-packet.c (unknown_pubkey_warning): If new flag is set do
 not print info message normally emitted inh verbose mode.
(can_handle_critical_notation, enum_sig_subpkt): Ditto.
(parse_signature, parse_key, parse_attribute_subpkts): Ditto.

Those messages are annoying because they might be emitted due to
parsing public keys which are latter not shows because the secret part
is missing. No functional regressions are expected because --verbose
should not change anything.

Note that this suppression is only done if no arguments are given to
the command; that is if a listing of the entire keyring is requested.
Thus to see the earnings anyway, a listing of a single or group of
keys can be requested.

• GnuPG-bug-id: T4627
• Signed-off-by: Werner Koch <wk@gnupg.org>