Bugfix: Fix rclone backend prematurely exiting when receiving SIGINT on Windows

On Windows, Restic now start the rclone process detached from the
restic console (similar to starting processes on a new process
group on Linux). Therefore, when Ctrl+C is pressed on the console
where restic runs, restic could gracefully clean up using clone,
and rclone won't exit prematurely leading to restic hanging up with
"stdio pipes closed" errors.

https://github.com/restic/restic/issue/3601
https://github.com/restic/restic/pull/3602