Before this change CTRL-C could come in to exit rclone which would
start the atexit actions running. The Fuse unmount then signals rclone
to exit which wasn't waiting for the already running atexit actions to
complete.
This change makes sure that if the atexit actions are started they
should be completed.
When running `rclone mount`, there were 2 signal handlers for `os.Interrupt`.
Those handlers would run concurrently and in some cases cause either unmount or `atexit.Run()` being skipped.
In addition `atexit.Run()` will get called in `resolveExitCode` to ensure cleanup on errors.
