`std::os::fd` defines types like `OwnedFd` and `RawFd` and is common
between Unix and non-Unix platforms that share a basic file-descriptor
concept. Rust currently uses this internally to simplify its own code,
but it would be useful for external users in the same way, so make it
public.

This means that `OwnedFd` etc. will all appear in three places, for
example on unix platforms:
 - `std::os::fd::OwnedFd`
 - `std::os::unix::io::OwnedFd`
 - `std::os::unix::prelude::OwnedFd`

Add `#[unstable(feature = "os_fd", issue = "98699")]` to the new
`pub use` declarations.