IE 8/9 Web Interface moving planes

Many thanks to 0xFACE for this fix. Hopefully those using Windows and
IE8/9 should now see some moving action.

net_io.c

@@ -721,6 +721,8 @@ int handleHTTPRequest(struct client *c, char *p) {
         "Content-Type: %s\r\n"
         "Connection: %s\r\n"
         "Content-Length: %d\r\n"
+        "Cache-Control: no-cache, must-revalidate\r\n"
+        "Expires: Sat, 26 Jul 1997 05:00:00 GMT\r\n"
         "\r\n",
         ctype,
         keepalive ? "keep-alive" : "close",