[3.7] bpo-34589: Add -X coerce_c_locale option; C locale coercion off by default (GH-9379)

* bpo-34589: Make _PyCoreConfig.coerce_c_locale private (GH-9371)

_PyCoreConfig:

* Rename coerce_c_locale to _coerce_c_locale
* Rename coerce_c_locale_warn to _coerce_c_locale_warn

These fields are now private (name prefixed by "_").

(cherry picked from commit 188ebfa475a6f6aa2d0ea14ca8e1fbe7865b6d27)

* bpo-34589: C locale coercion off by default (GH-9073)

Py_Initialize() and Py_Main() cannot enable the C locale coercion
(PEP 538) anymore: it is always disabled. It can now only be enabled
by the Python program ("python3).

test_embed: get_filesystem_encoding() doesn't have to set PYTHONUTF8
nor PYTHONCOERCECLOCALE, these variables are already set in the
parent.

(cherry picked from commit 7a0791b6992d420dc52536257f2f093851ed7215)

* bpo-34589: Add -X coerce_c_locale command line option (GH-9378)

Add a new -X coerce_c_locale command line option to control C locale
coercion (PEP 538).

(cherry picked from commit dbdee0073cf0b88fe541980ace1f650900f455cc)
diff --git a/Programs/python.c b/Programs/python.c
index 78e48f8..c7697fa 100644
--- a/Programs/python.c
+++ b/Programs/python.c
@@ -6,7 +6,7 @@
 int
 wmain(int argc, wchar_t **argv)
 {
-    return Py_Main(argc, argv);
+    return _Py_WindowsMain(argc, argv);
 }
 #else
 int