add Freebsd support

Adding entry to build masscan, and work on FreeBSD.
I have done regression test on my freebsd box, following.

% uname -a
FreeBSD arty 10.0-CURRENT FreeBSD 10.0-CURRENT #1 r255168: Tue Sep  3 19:27:55 JST 2013     user@arty:/usr/obj/usr/src/sys/ARTY  amd64

% gmake regress
bin/masscan --selftest
regression test: success!