Kernels higher than 2.6.10 don't support multiple --to arguments in
DNAT and SNAT targets.  At present, the error is somewhat vague:

# iptables -t nat -A foo -j SNAT --to 1.2.3.4 --to 2.3.4.5
iptables: Invalid argument

But if we want current iptables to work with kernels <= 2.6.10, we
cannot simply disallow this in all cases.

So the below patch adds kernel version checking to iptables, and
utilizes it in [DS]NAT.  Now, users will see a more informative error:

# iptables -t nat -A foo -j SNAT --to 1.2.3.4 --to 2.3.4.5
iptables v1.3.3: Multiple --to-source not supported

This generic infrastructure (shamelessly lifted from procps btw) may
come in handy in the future for other changes.

This fixes bugzilla #367.

Phil