Fix crash in strdup() when no default NAT engine is available.
Enable Mac OS X 10.10 feature detection

This is pointed at the 10.9 pf headers for testing, as
the 10.10 kernel sources are unavailable.
Merge pull request #32 from fix-macosx/macosx-yosemite

Support Mac OS X 10.10 by using 10.9 headers