mount: ensure atexit gets run on interrupt

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.

cmd/cmd.go

@@ -471,6 +471,7 @@ func initConfig() {
 }
 
 func resolveExitCode(err error) {
+	atexit.Run()
 	if err == nil {
 		os.Exit(exitCodeSuccess)
 	}