^C handling and breaks

[tonyg]

At the moment, ^C causes Racket to signal an exn:break in a random Syndicate actor. This is perfectly backwards: we want the ground-vm to catch such signals and engage in a protocol to negotiate shutdown with the contained actors.

@howell's suggestion is fundamentally sound: change run-ground from (in essence) its current (loop) to (sync (thread (lambda () (loop)))). The reason for this is the (undocumented?) fact that exn:break is only ever delivered to the main Racket thread.

Complications: terminating the loop when the outer thread terminates, i.e., ensure a bidirectional connection; and implementing the protocol to allow actors to control break-based termination. A sketch of the whole thing:

(let ((outer-thread (current-thread)))
  (with-handlers [(exn:break? (lambda (exn) (engage-in-protocol-with-contained-vm exn)))]
    (sync (thread (lambda () (loop outer-thread))))))

... where loop monitors outer-thread, terminating (without engaging in any kind of protocol) if it terminates.