gpg: Sort the signatures in standard key listings.

* g10/gpg.c (parse_list_options): Add "sort-sigs".
(main): Make it the default.
* g10/options.h (LIST_SORT_SIGS): New.
* g10/keylist.c (cmp_signodes): New.
(list_keyblock_print): Sort signatures and factor signature printing
code out to ...
(list_signature_print): new.

In particular together with --full-timestamps this makes it easier to
see the history of key signatures and their revocations. The
self-signatures are also printed first. To disable this

--list-options no-sort-sigs

can be used.

Also don't print the annoying "no recocation reason specified"
message.

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