#71 cleanup net/bpf.h include guards

[Tom Henderson]

From YunQiang Su:
Does some other main distribtution have bpf.h ? cygwin ? solaris ?aix etc?

It seems that only BSD system has net/bpf.h

Maybe we can do it like : http://predef.sourceforge.net/preos.html#sec12

#if !defined(__linux__)&&!defined(__APPLE__)
#include <net/bpf.h>
#endif

----------->>>>>>>>>>>>
#defined(__FreeBSD__) || defined(__FreeBSD_kernel__) ||
defined(__NetBSD__) || defined(__OpenBSD__)
#include <net/bpf.h>
#endif