Add handling for IPv6 extension header chains (addresses #531)

- The script-layer 'pkt_hdr' type is extended with a new 'ip6' field
  representing the full IPv6 header chain.

- The 'new_packet' event is now raised for IPv6 packets (addresses #523)

- A new event called 'ipv6_ext_header' is raised for any IPv6 packet
  containing extension headers.

- A new event called 'esp_packet' is raised for any packets using ESP
  ('new_packet' and 'ipv6_ext_header' events provide connection info,
  but that info can't be provided here since the upper-layer payload
  is encrypted).

- The 'unknown_protocol' weird is now raised more reliably when Bro
  sees a transport protocol or IPv6 extension header it can't handle.
  (addresses #522)

Still need to do IPv6 fragment reassembly and needs more testing.

src/PacketSort.cc

@@ -33,7 +33,7 @@ PacketSortElement::PacketSortElement(PktSrc* arg_src,
 
 		if ( ip_hdr->NextProto() == IPPROTO_TCP &&
 		      // Note: can't sort fragmented packets
-		     (ip_hdr->FragField() & 0x3fff) == 0 )
+		     ( ! ip_hdr->IsFragment() ) )
 			{
 			tcp_offset = hdr_size + ip_hdr->HdrLen();
 			if ( caplen >= tcp_offset + sizeof(struct tcphdr) )