* pythongh-157695: Parse also _Py_HAVE_xxx variables in sysconfig (python#158926)

Add parse_config_h() tests to test_sysconfig.

(cherry picked from commit 978a8be)

* pythongh-158893: Make os.strerror() thread-safe (python#158927)

Make os.strerror() thread-safe: use the reentrant strerror_r()
function if available.

* The configure script now checks if strerror_r() is supported.
* Add a stress test to test_free_threading.test_os (new module).
* Add a comment on decode_current_locale() assertion which fails if
  the input string is mutated.
* Add an assertion to _Py_DecodeLocale() to detect if the input
  string was mutated during the function call.

* pythongh-158893: Add internal _Py_strerror() function (python#158982)

Add a new internal _Py_strerror() function to Python/fileutils.c. It
uses strerror_r() if available, or use strerror() otherwise.

Replace all strerror(code) calls with _Py_strerror(code).

(cherry picked from commit 15dd735)
(cherry picked from commit c774d15)