PosixString.hsc: expose Fd

[domenkozar]

No description provided.

[hasufell]

Why?

Neither System.Posix.IO.ByteString, nor System.Posix.IO re-export them.

If we wanted, we should be consistent. I'm not sure how I generally feel about re-exporting base types.

[domenkozar]

Quite a few of functions have Fd in their type signature, so now I need to import it from System.IO if I want to use them.

The way I see it, it already exposes the API on the Fd so it just makes it convenient to have it as one import.

The same goes for SeekMode.

[hasufell]

I don't have a strong opinion, but it should be consistent.