fix signal handlers
fix signal handlers

Ignore-this: 202ae7a501ff1b97ae372bdc62381a64

- empty the signal mask if using sigaction
- reinstall the signal handler in the handler if using signal

darcs-hash:20120214091854-c41ad-0c0771ba54c7aa3bb0d411ba18b7cbbf1cf345b0

rneswold/lwt,mfp/lwt,gfxmonk/lwt,ocsigen/lwt,ocsigen/lwt,janestreet/lwt-async,fdopen/lwt,mirage/lwt,rneswold/lwt,c-cube/lwt,gfxmonk/lwt,rgrinberg/lwt,rgrinberg/lwt,c-cube/lwt,fdopen/lwt,mirage/lwt,rgrinberg/lwt,mfp/lwt,rgrinberg/lwt,ocsigen/lwt,rneswold/lwt,janestreet/lwt-async,c-cube/lwt