Proposed patch adds checks for null character in unicode path arguments of os functions on Windows. Null character is already tested on Unix, in bytes paths on Windows, and in unicode argument of _io.FileIO.

Removed private function _PyUnicode_HasNULChars(), because it is used only in two places and inlined code is simpler and more efficient.

The patch doesn't contain tests because I can't test them. But they should be simple, just pass a path with null character to os function.