exit REPL without exiting process

I want to embed a REPL and be able to exit the REPL without exiting the process.

Take this code as example:

```
express = require("express");
repl = require("repl");

app = express();

start = function(context) {
  r = repl.start("> ");
  for (k in context) {
    r.context[k] = context[k];
  }
  r.on('exit', function() {
    console.log("about to do some important stuff before exiting the REPL");
  });
};

app.get('/', function(req, res) {
  start({
    req: req,
    res: res
  });
  res.send("OK");
});

server = app.listen(3000, function() {
  console.log('Listening on port %d', server.address().port);
});
```

Without this small change, ctrl+d in the embedded REPL will exit the process completely, without even firing the REPL's exit event. With this PR, ctrl+d will exit the REPL but leave the process alive, whereas two `ctrl+c`s will exit the process as usual.

...but I think that's particular to your example actually, since you're hooking the exit signal.