Add --fast-dns-retry option.

This gives dnsmasq the ability to originate retries for upstream DNS
queries itself, rather than relying on the downstream client. This is
most useful when doing DNSSEC over unreliable upstream network. It
comes with some cost in memory usage and network bandwidth.

src/dnsmasq.h

@@ -755,11 +755,12 @@ struct frec {
   unsigned short new_id;
   int forwardall, flags;
   time_t time;
+  u32 forward_timestamp;
   unsigned char *hash[HASH_SIZE];
-#ifdef HAVE_DNSSEC 
-  int class, work_counter;
   struct blockdata *stash; /* Saved reply, whilst we validate */
   size_t stash_len;
+#ifdef HAVE_DNSSEC 
+  int class, work_counter;
   struct frec *dependent; /* Query awaiting internally-generated DNSKEY or DS query */
   struct frec *next_dependent; /* list of above. */
   struct frec *blocking_query; /* Query which is blocking us. */
@@ -1184,6 +1185,7 @@ extern struct daemon {
   int dump_mask;
   unsigned long soa_sn, soa_refresh, soa_retry, soa_expiry;
   u32 metrics[__METRIC_MAX];
+  int fast_retry_time;
 #ifdef HAVE_DNSSEC
   struct ds_config *ds;
   char *timestamp_file;
@@ -1404,6 +1406,7 @@ int hostname_order(const char *a, const char *b);
 int hostname_isequal(const char *a, const char *b);
 int hostname_issubdomain(char *a, char *b);
 time_t dnsmasq_time(void);
+u32 dnsmasq_milliseconds(void);
 int netmask_length(struct in_addr mask);
 int is_same_net(struct in_addr a, struct in_addr b, struct in_addr mask);
 int is_same_net_prefix(struct in_addr a, struct in_addr b, int prefix);
@@ -1463,6 +1466,7 @@ int send_from(int fd, int nowild, char *packet, size_t len,
 void resend_query(void);
 int allocate_rfd(struct randfd_list **fdlp, struct server *serv);
 void free_rfds(struct randfd_list **fdlp);
+int fast_retry(time_t now);
 
 /* network.c */
 int indextoname(int fd, int index, char *name);