* scd/command.c (hlp_keyinfo): Update.
(send_keyinfo): Add a USAGE argument.
* scd/scdaemon.h (send_keyinfo): Add a USAGE argument.
* scd/app-nks.c (set_usage_string): New.
(do_learn_status_core, do_readkey): Use set_usage_string.
(do_with_keygrip): Add USAGE to call send_keyinfo,
using set_usage_string.
* scd/app-openpgp.c (get_usage_string): New.
(send_keypair_info): Use get_usage_string.
(send_keyinfo_if_available): Add USAGE to call send_keyinfo,
using get_usage_string.
* scd/app-p15.c (set_usage_string): New.
(send_keypairinfo): Use set_usage_string.
(do_with_keygrip): Add USAGE to call send_keyinfo,
using set_usage_string.
* scd/app-piv.c (do_with_keygrip): Add USAGE to call send_keyinfo.