Support 464xlat on broadcast interfaces such as wifi.

This works by generating a random IID and then using the
IPV6_JOIN_ANYCAST socket option on the write-only raw socket to
configure an address on the interface.

Change-Id: Ieb885b7c54454988e2e4254a14b4213cba3bd791
diff --git a/setif.h b/setif.h
index 7f83f73..d31eed5 100644
--- a/setif.h
+++ b/setif.h
@@ -21,4 +21,7 @@
 int add_address(const char *ifname, int family, const void *address, int cidr, const void *broadcast);
 int if_up(const char *ifname, int mtu);
 
+int add_anycast_address(int sock, const struct in6_addr *addr, const char *interface);
+int del_anycast_address(int sock, const struct in6_addr *addr);
+
 #endif