--max-transfer - add new exit code (10)

It adds dedicated exit code (10) for --max-duration flag.

Rclone will exit with exit code 10 if the duration limit is reached.

It behaves in similar fashion as --max-transfer and exit code 8.

discussed on the forum:

https://forum.rclone.org/t/max-duration-option-is-triggering-exit-with-error/39917/6

lib/exitcode/exitcode.go

@@ -22,4 +22,6 @@ const (
 	TransferExceeded
 	// NoFilesTransferred everything succeeded, but no transfer was made.
 	NoFilesTransferred
+	// DurationExceeded is returned when transfer duration exceeded the quota.
+	DurationExceeded
 )