* 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.

Add a new _Py_INIT_EXIT() macro to be able to exit Python with an
exitcode using _PyInitError API. Rewrite function calls by
pymain_main() to use _PyInitError.

Changes:

* Remove _PyMain.err and _PyMain.status field
* Add _Py_INIT_EXIT() macro and _PyInitError.exitcode field.
* Rename _Py_FatalInitError() to _Py_ExitInitError().

Rename Include/internal/ headers:

* pycore_hash.h -> pycore_pyhash.h
* pycore_lifecycle.h -> pycore_pylifecycle.h
* pycore_mem.h -> pycore_pymem.h
* pycore_state.h -> pycore_pystate.h

Add missing headers to Makefile.pre.in and PCbuild:

* pycore_condvar.h.
* pycore_hamt.h
* pycore_pyhash.h

* Rename Include/internal/ header files:

  * pyatomic.h -> pycore_atomic.h
  * ceval.h -> pycore_ceval.h
  * condvar.h -> pycore_condvar.h
  * context.h -> pycore_context.h
  * pygetopt.h -> pycore_getopt.h
  * gil.h -> pycore_gil.h
  * hamt.h -> pycore_hamt.h
  * hash.h -> pycore_hash.h
  * mem.h -> pycore_mem.h
  * pystate.h -> pycore_state.h
  * warnings.h -> pycore_warnings.h

* PCbuild project, Makefile.pre.in, Modules/Setup: add the
  Include/internal/ directory to the search paths of header files.
* Update includes. For example, replace #include "internal/mem.h"
  with #include "pycore_mem.h".

Modify frozenmain.c to use _Py_InitializeFromConfig().

* Don't use "Python runtime" anymore to parse command line options or
  to get environment variables: pymain_init() is now a strict
  separation.
* Use an error message rather than "crashing" directly with
  Py_FatalError(). Limit the number of calls to Py_FatalError(). It
  prepares the code to handle errors more nicely later.
* Warnings options (-W, PYTHONWARNINGS) and "XOptions" (-X) are now
  only added to the sys module once Python core is properly
  initialized.
* _PyMain is now the well identified owner of some important strings
  like: warnings options, XOptions, and the "program name". The
  program name string is now properly freed at exit.
  pymain_free() is now responsible to free the "command" string.
* Rename most methods in Modules/main.c to use a "pymain_" prefix to
  avoid conflits and ease debug.
* Replace _Py_CommandLineDetails_INIT with memset(0)
* Reorder a lot of code to fix the initialization ordering. For
  example, initializing standard streams now comes before parsing
  PYTHONWARNINGS.
* Py_Main() now handles errors when adding warnings options and
  XOptions.
* Add _PyMem_GetDefaultRawAllocator() private function.
* Cleanup _PyMem_Initialize(): remove useless global constants: move
  them into _PyMem_Initialize().
* Call _PyRuntime_Initialize() as soon as possible:
  _PyRuntime_Initialize() now returns an error message on failure.
* Add _PyInitError structure and following macros:

  * _Py_INIT_OK()
  * _Py_INIT_ERR(msg)
  * _Py_INIT_USER_ERR(msg): "user" error, don't abort() in that case
  * _Py_INIT_FAILED(err)