Add the ability to specify destination port in DHCP-relay mode.

This change also removes a previous bug
where --dhcp-alternate-port would affect the port used
to relay _to_ as well as the port being listened on.
The new feature allows configuration to provide bug-for-bug
compatibility, if required. Thanks to Damian Kaczkowski
for the feature suggestion.

src/dnsmasq.h

@@ -1085,6 +1085,7 @@ struct dhcp_relay {
   union all_addr local, server;
   char *interface; /* Allowable interface for replies from server, and dest for IPv6 multicast */
   int iface_index; /* working - interface in which requests arrived, for return */
+  int port;        /* Port of relay we forward to. */
 #ifdef HAVE_SCRIPT
   struct snoop_record {
     struct in6_addr client, prefix;