[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/Lib/test/test_embed.py b/Lib/test/test_embed.py
index 2927458..79c34ec 100644
--- a/Lib/test/test_embed.py
+++ b/Lib/test/test_embed.py
@@ -267,9 +267,6 @@
         'malloc_stats': 0,
         'utf8_mode': 0,
 
-        'coerce_c_locale': 0,
-        'coerce_c_locale_warn': 0,
-
         'program_name': './_testembed',
         'argc': 0,
         'argv': '[]',
@@ -290,6 +287,8 @@
 
         '_disable_importlib': 0,
         'Py_FrozenFlag': 0,
+        '_coerce_c_locale': 0,
+        '_coerce_c_locale_warn': 0,
     }
 
     def check_config(self, testname, expected):