Move from one set of headers per major OS X release to one set of
headers per XNU release. Fetch the header files from Apple's official
Open Source site instead of GitHub in the fetchdeps developer target.
As a side effect, 10.6.x is now supported as well (untested), and proper
headers are used for 10.10.
Issue: #39
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