NAT64 support

This implements #721.  Includes documentation and some very basic tests.
Please refer to doc for further detail.

iterator/iterator.h

@@ -117,6 +117,18 @@ struct iter_env {
 	/** A flag to indicate whether or not we have an IPv4 route */
 	int supports_ipv4;
 
+	/** A flag to locally apply NAT64 to make IPv4 addrs into IPv6 */
+	int use_nat64;
+
+	/** NAT64 prefix address, cf. dns64_env->prefix_addr */
+	struct sockaddr_storage nat64_prefix_addr;
+
+	/** sizeof(sockaddr_in6) */
+	socklen_t nat64_prefix_addrlen;
+
+	/** CIDR mask length of NAT64 prefix */
+	int nat64_prefix_net;
+
 	/** A set of inetaddrs that should never be queried. */
 	struct iter_donotq* donotq;