Don't use getdtablesize() for closing existing fd's it only

represents the current limit.

Also don't call it every iteration of the loop.

bin/varnishd/mgt/mgt_main.c

@@ -530,7 +530,7 @@ main(int argc, char * const *argv)
 	 * Start out by closing all unwanted file descriptors we might
 	 * have inherited from sloppy process control daemons.
 	 */
-	for (o = getdtablesize(); o > STDERR_FILENO; o--)
+	for (o = sysconf(_SC_OPEN_MAX); o > STDERR_FILENO; o--)
 		(void)close(o);
 
 	VRND_Seed();

bin/varnishtest/vtc_process.c

@@ -203,7 +203,7 @@ process_start(struct process *p)
 		assert(dup2(p->fds[0], 0) == 0);
 		assert(dup2(out_fd, 1) == 1);
 		assert(dup2(err_fd, 2) == 2);
-		for (i = 3; i <getdtablesize(); i++)
+		for (i = sysconf(_SC_OPEN_MAX); i > STDERR_FILENO; i--)
 			(void)close(i);
 		AZ(execl("/bin/sh", "/bin/sh", "-c", VSB_data(cl), (char*)0));
 		exit(1);