Pause execution and return to CLI on ^C

Consider this a feature request with a prototype, I guess, because I get the following when I actually try to use this:

```
dlv> continue
new thread spawned 12773
new thread spawned 12774
^CStopped at: :269
Command failed: open : no such file or directory
dlv> 
```

Is this just what happens when it stops in the runtime guts rather than go-land?
There are merge confilcts here because this code is in the start() local function which does not exist anymore due to #12. If you can pull this logic up to right under the printv check it should be fine.

Also -- we would want this signal handler registered no matter what, and with this PR the start() function was only called when invoking a binary on behalf of the user, so it would miss the case of attaching to a running process.