timeout: fix bug in --preserve-status mode

Fix a bug where `timeout --preserve-status` was not correctly
preserving the status code of the child process if it timed out. When
that happens, the status code of the child process is considered to be
the signal number (in this case, `SIGTERM`). The exit status of
`timeout` is then 128 plus the numeric code associated with `SIGTERM`.

tests/by-util/test_timeout.rs

@@ -53,3 +53,14 @@ fn test_command_empty_args() {
         .fails()
         .stderr_contains("timeout: empty string");
 }
+
+#[test]
+fn test_preserve_status() {
+    new_ucmd!()
+        .args(&["--preserve-status", ".1", "sleep", "10"])
+        .fails()
+        // 128 + SIGTERM = 128 + 15
+        .code_is(128 + 15)
+        .no_stderr()
+        .no_stdout();
+}