[racket] Notification when window is closed

At Mon, 13 Aug 2012 07:57:56 -0600, Matthew Flatt wrote:
> Probably Racket should catch SIGTERM and SIGHUP and trigger calls to
> new handlers whose default actions call the exit handler.

On second thought, those should turn into variants of `exn:break', like
SIGINT handling.

I'll add `exn:break:hang-up' and `exn:break:terminate'. The default
uncaught exception handler will call `exit' (after displaying the
exception) when it sees an instance of either.