client: support --status-interval option in client/server mode

client: support --status-interval option in client/server mode

This fixes "FIO client/server periodic stats" for non JSON output
(the reporter is using JSON output).
https://www.spinics.net/lists/fio/msg07204.html

Mark -L/--status-interval option with FIO_CLIENT_FLAG, so that the
command line string is sent to the server side.

The client can now control the server to periodically show status,
and each status is sent to the client via fio_server_text_output()
(FIO_NET_CMD_TEXT socket event) in log_info_buf().

Reported-by: Mark Beierl <Mark.Beierl@dell.com>
Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>