Impove cache behaviour for TCP connections.

For ease of implementaion, dnsmasq has always forked a new process to
handle each incoming TCP connection. A side-effect of this is that any
DNS queries answered from TCP connections are not cached: when TCP
connections were rare, this was not a problem.  With the coming of
DNSSEC, it's now the case that some DNSSEC queries have answers which
spill to TCP, and if, for instance, this applies to the keys for the
root then those never get cached, and performance is very bad.  This
fix passes cache entries back from the TCP child process to the main
server process, and fixes the problem.

src/dnsmasq.h

@@ -1091,6 +1091,8 @@ extern struct daemon {
   size_t packet_len;       /*      "        "        */
   struct randfd *rfd_save; /*      "        "        */
   pid_t tcp_pids[MAX_PROCS];
+  int tcp_pipes[MAX_PROCS];
+  int pipe_to_parent;
   struct randfd randomsocks[RANDOM_SOCKS];
   int v6pktinfo; 
   struct addrlist *interface_addrs; /* list of all addresses/prefix lengths associated with all local interfaces */
@@ -1152,6 +1154,7 @@ struct crec *cache_find_by_name(struct crec *crecp,
 				char *name, time_t now, unsigned int prot);
 void cache_end_insert(void);
 void cache_start_insert(void);
+int cache_recv_insert(time_t now, int fd);
 struct crec *cache_insert(char *name, struct all_addr *addr,
 			  time_t now, unsigned long ttl, unsigned short flags);
 void cache_reload(void);
@@ -1174,6 +1177,8 @@ void blockdata_init(void);
 void blockdata_report(void);
 struct blockdata *blockdata_alloc(char *data, size_t len);
 void *blockdata_retrieve(struct blockdata *block, size_t len, void *data);
+struct blockdata *blockdata_read(int fd, size_t len);
+void blockdata_write(struct blockdata *block, size_t len, int fd);
 void blockdata_free(struct blockdata *blocks);
 #endif