aix getpgid fails when called from parent and parent and child have different session

aix - getpgid fails with EPERM when called from parent process and the parent and child have different session (man page also states this) 
Fix is to create a new process group and have same session.
so I think down here you can:

```ruby
if (kill_return_val != 0)
   # we killed something, which is wrong, so complain about it
  [ .. stuff .. ]
else
    # on AIX, instead of an exception we get > 0 back from kill when it fails, which in this case we
    # ignore as being successful.
end
```

trying to mimic what Linux does on AIX inside of a spec test just makes my head hurt trying to understand why...