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.
Lwt_glib: use -1 (not -1000) as "no timeout"

On BSD systems (including OS X), passing -1000 to poll results in
EINVAL.

Reported by Anders F Björklund.

Fixes #25 on OS X.