scd: Fix DEVINFO, allowing no clients which watch the change.
* scd/app.c [POSIX] (struct mrsw_lock): Add notify_watchers. (card_list_signal): Only when watchers wait, kick by write(2). (card_list_wait): Increment/decrement notify_watchers field.
- GnuPG-bug-id: T7151
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>