Previously sending packets on a loopback interface via bpf failed
because the packets are treated as AF_UNSPEC by bpf and the loopback
interface couldn't handle such packets.
This fix enables user programs to prepend a protocol family (AF_INET or
AF_INET6) to a payload. bpf interprets it and treats a packet as so,
not just AF_UNSPEC. The protocol family is encoded as 4 bytes, host byte
order as per DLT_NULL in the specification(*).
(*) https://www.tcpdump.org/linktypes.html
Proposed on tech-net and tech-kern
