Update decline address handling in DHCPv6 for new multi-address world.

When dhcp-host options can have many IPv6 addresses, we need
to deal with one of them being declined by a client. The other
addresses are still valid.

It seems that this logic never worked, even with only one address, since
the DECLINED flag was never tested.

src/dnsmasq.h

@@ -384,10 +384,12 @@ struct ds_config {
 #define ADDRLIST_REVONLY  4
 #define ADDRLIST_PREFIX   8
 #define ADDRLIST_WILDCARD 16
+#define ADDRLIST_DECLINED 32
 
 struct addrlist {
   union all_addr addr;
-  int flags, prefixlen; 
+  int flags, prefixlen;
+  time_t decline_time;
   struct addrlist *next;
 };