_Py_char2wchar() callers usually need the result size in characters. Since it's
trivial to compute it in _Py_char2wchar() (O(1) whereas wcslen() is O(n)), add
an option to get it.
* Don't define _Py_wstat() on Windows, Windows has its own _wstat() function
with a different API (the stat buffer has another type)
* Include windows.h
* _Py_fopen() and _Py_stat() come from Python/import.c
* (_Py)_wrealpath() comes from Python/sysmodule.c
* _Py_char2wchar(), _Py_wchar2char() and _Py_wfopen() come from Modules/main.c
* (_Py)_wstat(), (_Py)_wgetcwd(), _Py_wreadlink() come from Modules/getpath.c