BUG/MINOR: cli: use global.maxsock and not maxfd to list all FDs

The "show fd" command on the CLI doesn't list the last FD in use since
it doesn't include maxfd. We don't need to use maxfd here anyway as
global.maxsock will do the job pretty well and removes this dependency.
This patch may be backported to 1.8.

src/cli.c

@@ -772,7 +772,7 @@ static int cli_io_handler_show_fd(struct appctx *appctx)
 	/* we have two inner loops here, one for the proxy, the other one for
 	 * the buffer.
 	 */
-	while (fd < maxfd) {
+	while (fd < global.maxsock) {
 		struct fdtab fdt;
 		struct listener *li = NULL;
 		struct server *sv = NULL;