Issue #48.
The fix is actually very small, but the issue prompted me to create the
hairpinning unit test, which got a little messy.
Also, I fixed a bunch of incorrect header guards. Some of them didn't exist,
and others were incorrectly named.
Merge branch 'master' of github.com:NICMx/NAT64

Conflicts:
	mod/core.c
	mod/nf_hook.c