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Add 'loopback' header parsing to the sniff mode.

On darwin, pcap returns packets from the loopback device
with a 4 byte 'family' number, instead of the dummy eth
header we get on linux. Add support for parsing this
instead. The build now needs to define either 'LOOP' or
'ETH' to control which is expected.

I don't know if there's a way to ask pcap about this,
but wireshark manages to handle it correctly.

Also hardcode the sniff device to 'lo0' as required by
darwin instead of linux 'lo'. That's where I'm currently
testing.

--- a/src/opusrtp.c

+++ b/src/opusrtp.c

@@ -273,6 +273,11 @@

   int type;

 } eth_header;

+#define LOOP_HEADER_LEN 4

+typedef struct {

+  int family;

+} loop_header;

+

 #define IP_HEADER_MIN 20

 typedef struct {

   int version;

@@ -329,6 +334,21 @@

   return 0;

 }

+/* used by the darwin loopback interface, at least */

+int parse_loop_header(const unsigned char *packet, int size, loop_header *loop)

+{

+  if (!packet || !loop) {

+    return -2;

+  }

+  if (size < LOOP_HEADER_LEN) {

+    fprintf(stderr, "Packet too short for loopback\n");

+    return -1;

+  }

+  loop->family = rbe32(packet);

+

+  return 0;

+}

+

 int parse_ip_header(const unsigned char *packet, int size, ip_header *ip)

 {

   if (!packet || !ip) {

@@ -602,6 +622,7 @@

   const unsigned char *packet;

   int size;

   eth_header eth;

+  loop_header loop;

   ip_header ip;

   udp_header udp;

   rtp_header rtp;

@@ -611,6 +632,7 @@

   packet = data;

   size = header->caplen;

+#if ETH

   if (parse_eth_header(packet, size, &eth)) {

     fprintf(stderr, "error parsing eth header\n");

     return;

@@ -624,6 +646,15 @@

           eth.dst[3], eth.dst[4], eth.dst[5]);

   packet += ETH_HEADER_LEN;

   size -= ETH_HEADER_LEN;

+#elif LOOP

+  if (parse_loop_header(packet, size, &loop)) {

+    fprintf(stderr, "error parsing loopback header\n");

+    return;

+  }

+  fprintf(stderr, "  loopback family %d\n", loop.family);

+  packet += LOOP_HEADER_LEN;

+  size -= LOOP_HEADER_LEN;

+#endif

   if (parse_ip_header(packet, size, &ip)) {

     fprintf(stderr, "error parsing ip header\n");

@@ -790,7 +821,7 @@

       case 0:

         if (!strcmp(long_options[i].name, "sniff")) {

 #ifdef HAVE_PCAP

-          sniff("lo");

+          sniff("lo0");

           return 0;

 #else

           fprintf(stderr, "pcap support disabled, sorry.\n");