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

Previously, pressing Ctrl+C in a Windows console where restic was running with
rclone as the backend would cause rclone to exit prematurely due to getting a
`SIGINT` signal at the same time as restic. Restic would then wait for a long
time for time with "unexpected EOF" and "rclone stdio connection already closed"
errors.

This has now been fixed by restic starting the rclone process detached from the
console restic runs in (similar to starting processes in a new process group on
Linux), which enables restic to gracefully clean up rclone (which now never gets
the `SIGINT`).

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