Handle binding upstream servers to an interface

(--server=1.2.3.4@eth0) when the named interface
is destroyed and recreated in the kernel.

src/dnsmasq.h

@@ -488,6 +488,7 @@ struct serverfd {
   int fd;
   union mysockaddr source_addr;
   char interface[IF_NAMESIZE+1];
+  unsigned int ifindex, used;
   struct serverfd *next;
 };