agent: Better serialization for scdaemon access.

* agent/call-scd.c (unlock_scd): Move lock before accessing IN_USE.
(wait_child_thread): Add log_info for Windows, and fixed log_error
message.

The old code is still valid with cooperate threads, but this is
better.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>