bpo-32030: Simplify _PyCoreConfig_INIT macro (#4728)

* Simplify _PyCoreConfig_INIT, _PyMainInterpreterConfig_INIT,
  _PyPathConfig_INIT macros: no need to set fields to 0/NULL, it's
  redundant (the C language sets them to 0/NULL for us).
* Fix typo: pymain_run_statup() => pymain_run_startup()
* Remove a few XXX/TODO