Lwt_glib: report disconnected FDs as ready and release runtime during poll

If a HUP revent caused poll to return, there's no point ignoring it and polling again; it will still be disconnected. Instead, mark the FD as ready so the problem can be dealt with.

Also, release the OCaml runtime system while waiting so that other threads can make progress.

Fixes #25.

(note: not tested on Windows, only on Linux)