g10: Add "key-attr" command for --card-edit.
* g10/card-util.c (key_attr): New explicit command. (generate_card_keys, card_generate_subkey): Don't ask key attr change. (card_edit): Add for cmdKEYATTR.
- GnuPG-bug-id: T3781
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>