exit_with_usage() returns 1 only in case of error

"screen -h|--help" should NOT return 1

Sync with screen-v4: 7908c33f73

src/screen.c

@@ -269,8 +269,9 @@ static void exit_with_usage(char *myname, char *message, char *arg)
 		printf("\nError: ");
 		printf(message, arg);
 		printf("\n");
+		exit(1);
 	}
-	exit(1);
+	exit(0);
 }
 
 int main(int argc, char **argv)