New patch based on review feedback - main change is to _Py_fstat to use errno on Windows instead of GetLastError().

Where to put the invalid parameter handler is still an open question. Victor is not keen on adding a new file, while I don't really want it to be outside of the PC/ folder. msvcrtmodule.c is next best IMHO, but since this won't be getting a Python API it still seems wrong.

Alternatively, I can rename invalid_parameter_handler.c to something like "PC/crtutils.c" so it stands out as a multi-purpose file?