bpo-36444: Add _PyCoreConfig._init_main (GH-12572)

* Add _PyCoreConfig._init_main: if equals to zero,
  _Py_InitializeFromConfig() doesn't call
  _Py_InitializeMainInterpreter().
* Add interp_p parameter to _Py_InitializeFromConfig().
* pymain_init() now calls _Py_InitializeFromConfig().
* Make _Py_InitializeCore() private.
diff --git a/Include/cpython/pylifecycle.h b/Include/cpython/pylifecycle.h
index 1db1d21..496dcb2 100644
--- a/Include/cpython/pylifecycle.h
+++ b/Include/cpython/pylifecycle.h
@@ -20,9 +20,6 @@
 PyAPI_FUNC(_PyInitError) _Py_PreInitializeFromConfig(
     const _PyCoreConfig *coreconfig);
 
-PyAPI_FUNC(_PyInitError) _Py_InitializeCore(
-    const _PyCoreConfig *config,
-    PyInterpreterState **interp);
 PyAPI_FUNC(int) _Py_IsCoreInitialized(void);
 
 
@@ -32,7 +29,8 @@
 /* Initialization and finalization */
 
 PyAPI_FUNC(_PyInitError) _Py_InitializeFromConfig(
-    const _PyCoreConfig *config);
+    const _PyCoreConfig *config,
+    PyInterpreterState **interp_p);
 PyAPI_FUNC(void) _Py_NO_RETURN _Py_ExitInitError(_PyInitError err);
 
 /* Py_PyAtExit is for the atexit module, Py_AtExit is for low-level