GUI PuTTY: fix format-string goof on connect failure.

Introduced by 61f3e3e29, as part of my periodic efforts to make the
GTK front end work usefully on OS X: the 'Unable to open connection to
[host]: [error]' message box is accidentally passed through two layers
of printf format-string parsing, the second of which has no argument
list. So if you pass in a host name like '%s' on the command line,
bad things will happen when that error message is constructed.

This happened because in that commit I changed a call to
fatal_message_box() into a call to connection_fatal(), without
noticing that the latter was printf-style variadic and the former
wasn't. On the plus side, that means now I can remove the explicit
dupprintf/free around the error message.

unix/gtkwin.c

@@ -5085,11 +5085,10 @@ static void start_backend(GtkFrontend *inst)
                          conf_get_bool(inst->conf, CONF_tcp_keepalives));
 
     if (error) {
-	char *msg = dupprintf("Unable to open connection to %s:\n%s",
+	seat_connection_fatal(&inst->seat,
+                              "Unable to open connection to %s:\n%s",
 			      conf_dest(inst->conf), error);
 	inst->exited = true;
-	seat_connection_fatal(&inst->seat, msg);
-	sfree(msg);
         return;
     }