Rationalise DNS TCP buffer use.

This fixes the plethora of 64k buffers that got allocated when doing
DNSSEC over TCP.

By using the UDP buffer is pass the query into tcp_talk() and
allowing tcp_talk to allocate its output buffer one the size
of the reply is known, we only need to allocate as much memory as
is required. The final reply to the TCP query still needs the 64k
buffer because answer_request() and answer_auth() are not capable
of extending their output buffers.

src/dnsmasq.c

@@ -1980,11 +1980,11 @@ static void do_tcp_connection(struct listener *listener, time_t now, int slot)
   pid_t p;
   union mysockaddr tcp_addr;
   socklen_t tcp_len = sizeof(union mysockaddr);
-  unsigned char *buff;
   struct server *s; 
   int flags, auth_dns = 0;
   struct in_addr netmask;
   int pipefd[2];
+  struct iovec tcpbuff;
 #ifdef HAVE_LINUX_NETWORK
   unsigned char a = 0;
 #endif
@@ -2159,10 +2159,8 @@ static void do_tcp_connection(struct listener *listener, time_t now, int slot)
   if ((flags = fcntl(confd, F_GETFL, 0)) != -1)
     while(retry_send(fcntl(confd, F_SETFL, flags & ~O_NONBLOCK)));
 
-  buff = tcp_request(confd, now, &tcp_addr, netmask, auth_dns);
-	      
-  if (buff)
-    free(buff);
+  tcp_request(confd, now, &tcpbuff, &tcp_addr, netmask, auth_dns);
+  free(tcpbuff.iov_base);
   
   for (s = daemon->servers; s; s = s->next)
     if (s->tcpfd != -1)