bpo-36763: Add _Py_InitializeMain() (GH-13362)

* Add a private _Py_InitializeMain() function.
* Add again _PyCoreConfig._init_main.
* _Py_InitializeFromConfig() now uses _init_main to decide
  if _Py_InitializeMainInterpreter() should be called.
* _PyCoreConfig: rename _frozen to pathconfig_warnings, its value is
  now the opposite of Py_FrozenFlag.
* Add an unit test for _init_main=0 and _Py_InitializeMain().
diff --git a/Python/frozenmain.c b/Python/frozenmain.c
index a777576..f2499ef 100644
--- a/Python/frozenmain.c
+++ b/Python/frozenmain.c
@@ -40,7 +40,7 @@
     }
 
     _PyCoreConfig config = _PyCoreConfig_INIT;
-    config._frozen = 1;   /* Suppress errors from getpath.c */
+    config.pathconfig_warnings = 0;   /* Suppress errors from getpath.c */
 
     if ((p = Py_GETENV("PYTHONINSPECT")) && *p != '\0')
         inspect = 1;