bpo-38234: Fix _PyConfig_InitPathConfig() (GH-16335)

bpo-38234: Fix _PyConfig_InitPathConfig() (GH-16335)

* _PyConfig_InitPathConfig() now starts by copying the global path
  configuration, and then override values set in PyConfig.
* _PyPathConfig_Calculate() implementations no longer override
  _PyPathConfig fields which are already computed. For example,
  if _PyPathConfig.prefix is not NULL, leave it unchanged.
* If Py_SetPath() has been called, _PyConfig_InitPathConfig() doesn't
  call _PyPathConfig_Calculate() anymore.
* _PyPathConfig_Calculate() no longer uses PyConfig,
  except to initialize PyCalculatePath structure.
* pathconfig_calculate(): remove useless temporary
  "_PyPathConfig new_config" variable.
* calculate_module_search_path(): remove hack to workaround memory
  allocation failure, call Py_FatalError() instead.
* Fix get_program_full_path(): handle memory allocation failure.