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.
We can't really have if-else here, since Process.kill() raises an exception in linux platforms whereas in case of AIX it does not raise an exception and hence we have to assert. @lamont-granquist ^