Improve connection handling when talking to TCP upsteam servers.

Specifically, be prepared to open a new connection when we
want to make multiple queries but the upstream server accepts
fewer queries per connection.

src/dnsmasq.h

@@ -485,6 +485,7 @@ union mysockaddr {
 #define SERV_FROM_FILE      4096  /* read from --servers-file */
 #define SERV_LOOP           8192  /* server causes forwarding loop */
 #define SERV_DO_DNSSEC     16384  /* Validate DNSSEC when using this server */
+#define SERV_GOT_TCP       32768  /* Got some data from the TCP connection */
 
 struct serverfd {
   int fd;