os: enable the close-on-exec flag for openFdAt

There's a race here with fork/exec, enable the close-on-exec flag
for the new file descriptor.

Fixes #33405

Change-Id: If95bae97a52b7026a930bb3427e47bae3b0032ac
Reviewed-on: https://go-review.googlesource.com/c/go/+/188537
Run-TryBot: Baokun Lee <nototon@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/os/removeall_at.go

@@ -173,7 +173,7 @@ func openFdAt(dirfd int, name string) (*File, error) {
 	var r int
 	for {
 		var e error
-		r, e = unix.Openat(dirfd, name, O_RDONLY, 0)
+		r, e = unix.Openat(dirfd, name, O_RDONLY|syscall.O_CLOEXEC, 0)
 		if e == nil {
 			break
 		}