Add --port-limit option.

By default, when sending a query via random ports to multiple upstream servers or
retrying a query dnsmasq will use a single random port for all the tries/retries.
This option allows a larger number of ports to be used, which can increase robustness
in certain network configurations. Note that increasing this to more than
two or three can have security and resource implications and should only
be done with understanding of those.

src/dnsmasq.h

@@ -1139,6 +1139,7 @@ extern struct daemon {
   int log_fac; /* log facility */
   char *log_file; /* optional log file */
   int max_logs;  /* queue limit */
+  int randport_limit; /* Maximum number of source ports for query. */
   int cachesize, ftabsize;
   int port, query_port, min_port, max_port;
   unsigned long local_ttl, neg_ttl, max_ttl, min_cache_ttl, max_cache_ttl, auth_ttl, dhcp_ttl, use_dhcp_ttl;