bpo-42260: Main init modify sys.flags in-place (GH-23150)

When Py_Initialize() is called twice, the second call now updates
more sys attributes for the configuration, rather than only sys.argv.

* Rename _PySys_InitMain() to _PySys_UpdateConfig().
* _PySys_UpdateConfig() now modifies sys.flags in-place, instead of
  creating a new flags object.
* Remove old commented sys.flags flags (unbuffered and skip_first).
* Add private _PySys_GetObject() function.
* When Py_Initialize(), Py_InitializeFromConfig() and