Support pf rdr on Mac OS X 10.7, 10.8 and 10.9 by including the missing
Apple headers in the source tree and enable private Apple code. Since
we are using an interface marked private by Apple, this code is very
experimental.
Issue: #15
Reported by: Amit Chowdhary
For Linux netfilter, IPV6_ORIGINAL_DST and SOL_IPV6 are defined to
SO_ORIGINAL_DST and SOL_IP respectively if they are not defined by the
system headers (they aren't defined on vanilla kernels). Undefine these
compatibility definitions after use, in order not to mess up the
diagnostic output of nat_version().
