Add --vpn (-X) flag to support sending IP pkts

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).

src/send.c

@@ -288,7 +288,7 @@ int send_run(int sock)
 				zconf.probe_module->print_packet(stdout, buf);
 			} else {
 					int l = zconf.probe_module->packet_length;
-					int rc = sendto(sock, buf, 
+					int rc = sendto(sock, buf + zconf.send_ip_pkts*sizeof(struct ethhdr),
 							l, 0,
 							(struct sockaddr *)&sockaddr,
 							sizeof(struct sockaddr_ll));