When rclone received a SIGINT (Ctrl+C) or SIGTERM signal while an atexit
function is registered it always terminated with status code 0. Unix
convention is to exit with a non-zero status code. Often it's
`128 + int(signum), but at least not zero.
With this change fatal signals handled by the `atexit` package cause
a non-zero exit code. On Unix systems it's `128 + int(signum)` while
on other systems, such as Windows, it's always 2 ("error not otherwise
categorised").
Resolves#5437.
Signed-off-by: Michael Hanselmann <public@hansmi.ch>
