agent: Handle SCD DEVINFO --watch command in a special way.

* agent/call-scd.c (devinfo_watch_thread): New.
(agent_card_devinfo): New.
(agent_card_scd): Call agent_card_devinfo when it's
DEVINFO_WATCH_COMMAND.

(cherry picked from commit b3f1f2cd192b71f2c19ec6f871ece0c175d46db0)

• GnuPG-bug-id: T7151
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>