When a user specifies a `-i tun0` flag for a VPN interface,
this interface expects raw IP packets, not ethernet frames.
The --vpn option allows you to send only IP packets to the
specified interface, and also performs some hacks on the receive
end to ensure we can process the results.
TODO: malloc a fake ethernet header for probe_modules.process_packet
(or better yet, make process_packet always take an IP packet, they
all seem to be implicitly doing that anyway).
