Use O_NOFOLLOW_ANY on macOS

According to [this blog post](https://blog.malwarebytes.com/mac/2021/08/macos-11s-hidden-security-improvements/#O_NOFOLLOW_ANY), macos 11 has a new `O_NOFOLLOW_ANY` flag that causes `open` to fail if any path component is a symlink. Assuming symlinks are relatively rare, cap-std could use this as a fast path -- check that the path contains no explicit `..` and then open with `O_NOFOLLOW_ANY`; if that succeeds, then we're done. If either of those fails, open with the slow path.