Add FD_CLOEXEC and Windows equivalent if any

We now have O_CLOEXEC as a constant. We need FD_CLOEXEC for the fcntl version of it.

Windows defines O_NOINHERIT instead of O_CLOEXEC. If it has a similar equivalent to FD_CLOEXEC we should define that as well.

I think these just go in Fcntl since they're defined there (at least on MacOS).