Improve availability of IPv6 flow label in connection records.

Without this change, flow labeling of connections over IPv6 are
only available in the per-packet types of events (e.g. new_packet)
in which header fields can be inspected, but now minimal tracking
of the most recent flow label is done internally and that's available
per-connection for all events that use connection record arguments.

Specifically, this adds a "flow_label" field to the "endpoint" record
type, which is used for both the "orig" and "resp" fields of
"connection" records.  The new "connection_flow_label_changed" event
also allows tracking of changes in flow labels: it's raised each time
one direction of the connection starts using a different label.

src/IP.h

@@ -524,6 +524,12 @@ public:
 	int DF() const
 		{ return ip4 ? ((ntohs(ip4->ip_off) & 0x4000) != 0) : 0; }
 
+	/**
+	 * Returns value of an IPv6 header's flow label field or 0 if it's IPv4.
+	 */
+	uint32 FlowLabel() const
+		{ return ip4 ? 0 : (ntohl(ip6->ip6_flow) & 0x000fffff); }
+
 	/**
 	 * Returns number of IP headers in packet (includes IPv6 extension headers).
 	 */