two-node tool does not properly kill processes when closed

```
When closing the two-node tool after using it, processes such as 'ping -R' may
hang on some kernels, and the process is not killed as it should be when
closing the dialog.

This can lead to the GUI freezing. The workaround is to look at 'ps -aef'
output and manually kill the ping/traceroute process.

While we're here, may want to use tracepath instead of traceroute, as that may
work better (and is included by default) on modern distros.
```

Original issue reported on code.google.com by `ahrenh...@gmail.com` on 22 May 2012 at 5:35