Use `HandleOrNull` and `HandleOrInvalid` in the Windows FFI bindings.

Use `HandleOrNull` and `HandleOrInvalid` in the Windows FFI bindings.

Use the new `HandleOrNull` and `HandleOrInvalid` types that were introduced
as part of [I/O safety] in a few functions in the Windows FFI bindings.

This factors out an `unsafe` block and two `unsafe` function calls in the
Windows implementation code.

And, it helps test `HandleOrNull` and `HandleOrInvalid`, which indeed turned
up a bug: `OwnedHandle` also needs to be `#[repr(transparent)]`, as it's
used inside of `HandleOrNull` and `HandleOrInvalid` which are also
`#[repr(transparent)]`.

[I/O safety]: https://github.com/rust-lang/rust/issues/87074