Fix implementation of Lwt_unix.socketpair.
Fix implementation of Lwt_unix.socketpair.

Fixes #120.

rgrinberg/lwt,c-cube/lwt,c-cube/lwt,rgrinberg/lwt,rneswold/lwt,mfp/lwt,ocsigen/lwt,rgrinberg/lwt,mfp/lwt,rgrinberg/lwt,c-cube/lwt,rneswold/lwt,ocsigen/lwt,rneswold/lwt,ocsigen/lwt