nwfilter: check for inverted ctdir

nwfilter: check for inverted ctdir

Linux netfilter at some point (Linux 2.6.39) inverted the meaning of the
'--ctdir reply' and newer netfilter implementations now expect
'--ctdir original' instead and vice-versa.
We check for the kernel version and assume that all Linux kernels with version
2.6.39 have the newer inverted logic.

Any distro backporting the Linux kernel patch that inverts the --ctdir logic
(Linux commit 96120d86f) must also backport this patch for Linux and
adapt the kernel version being tested for.

Signed-off-by: Stefan Berger <stefanb@linux.vnet.ibm.com>