Mac OS X 10.10 Support

Minimal set of changes to support Mac OS X 10.10

- A fix for a crash that occurs when no NAT engine is available; this came up before I enabled -DHAVE_PF
- Enabling the existing Mavericks code for use on Yosemite.

The xnu source for 10.10 isn't available, but I can confirm the sslsplit does work on 10.10 when build against the 10.9 pf headers.