bpo-32030: _PyPathConfig_Init() sets home and program_name (#4673)

_PyPathConfig_Init() now also initialize home and program_name:

* Rename existing _PyPathConfig_Init() to _PyPathConfig_Calculate().
  Add a new _PyPathConfig_Init() function in pathconfig.c which
  handles the _Py_path_config variable and call
  _PyPathConfig_Calculate().
* Add home and program_name fields to _PyPathConfig.home
* _PyPathConfig_Init() now initialize home and program_name
  from main_config
* Py_SetProgramName(), Py_SetPythonHome() and Py_GetPythonHome() now
  calls Py_FatalError() on failure, instead of silently ignoring
  failures.
* config_init_home() now gets directly _Py_path_config.home to only
  get the value set by Py_SetPythonHome(), or NULL if
  Py_SetPythonHome() was not called.
* config_get_program_name() now gets directly
  _Py_path_config.program_name to only get the value set by
  Py_SetProgramName(), or NULL if Py_SetProgramName() was not called.
* pymain_init_python() doesn't call Py_SetProgramName() anymore,
  _PyPathConfig_Init() now always sets the program name
* Call _PyMainInterpreterConfig_Read() in
  pymain_parse_cmdline_envvars_impl() to control the memory allocator
* C API documentation: it's no more safe to call Py_GetProgramName()
  before Py_Initialize().