Ignore status of killUser() process on darwin.

Quoting from the man page of *kill(2)*:

```
If pid is -1:
    If the user has super-user privileges, the signal is
    sent to all processes excluding system processes and the
    process sending the signal. If the user is not the super
    user, the signal is sent to all processes with the same
    uid as the user, excluding the process sending the
    signal. No error is returned if any process could be
    signaled.
```

Unfortunately this is not the case and the process sending the signal is killed as well, so we can't assume that wait() will return 0 if the process drops dead in-between.