typing: add type hints to the common posix/windows platform functions These are done in sync because some platforms have empty implementations, and it isn't obvious what the types should be without examining the other. We want the types aligned, so @overload definitions that differ aren't generated. The only differences here are the few methods that unconditionally raise an error are marked as `NoReturn`, which doesn't seem to bother pytype. A couple of the posix module functions needed to be updated with a modern ternary operator, because pytype seems to want to use the type of the second object in the old `return x and y` style.