fix insignificant off-by-one in child file descriptor close code

The name CLOSE_FD_UP_TO implies that the value is included.

Also this is the semantics of MCH_TrackHighFd() and relevant for
 #define CLOSE_FD_UP_TO mgt_max_fd

bin/varnishd/mgt/mgt_child.c

@@ -344,7 +344,7 @@ mgt_launch_child(struct cli *cli)
 		 */
 		closelog();
 
-		for (i = STDERR_FILENO + 1; i < CLOSE_FD_UP_TO; i++) {
+		for (i = STDERR_FILENO + 1; i <= CLOSE_FD_UP_TO; i++) {
 			if (vbit_test(fd_map, i))
 				continue;
 			if (close(i) == 0)