gpg: New list-option --show-unusable-sigs.

* g10/options.h (LIST_SHOW_UNUSABLE_SIGS): New.
* g10/gpg.c (parse_list_options): Add "show-unusable-sigs".
* g10/keydb.h (keyid_eq): New.
(pk_is_primary): New.
* g10/keylist.c (list_signature_print): Early return for weak key
signatures.  Print "self-signature" instead of user-id.
(list_keyblock_print): Simplify and always set self-sig node flag.

This patch avoid the printing of often hundreds of "Invalid digest
algorithm" notices during key signature listings if those key
signatures were done with SHA1. The new option can be used to revert
the behaviour.

We now also print "[self-signature]" with --check-sigs or --list-sigs
instead of the primary user id. This makes such listing easier to read.