Test all 64 bits of sender's link-local addresses.

Link-local addresses are defined as being in fe80::/10, but they always
start with 54 bits of zeroes.  Test the whole first byte.

message.c

@@ -135,8 +135,7 @@ parse_packet(const unsigned char *from, struct network *net,
     unsigned char router_id[8], v4_prefix[16], v6_prefix[16],
         v4_nh[16], v6_nh[16];
 
-
-    if(from[0] != 0xFE || (from[1] & 0xC0) != 0x80) {
+    if(!in_prefix(from, ll_prefix, 64)) {
         fprintf(stderr, "Received packet from non-local address %s.\n",
                 format_address(from));
         return;