os: os.Root: use O_NOFOLLOW_ANY on macOS

[neild]

macOS 11 and later's openat supports an O_NOFOLLOW_ANY flag which causes resolution to fail if any path component contains a symlink. We can use this to implement a fast-path for os.Root operations in the common case when the path does not contain a symlink.

[tpaschalis]

I'm thinking of taking a stab at this, so wanted to verify/brainstorm a few things before jumping in.

1. With a glance I didn't see any cases where we conditionally use features for a specific macOS versions but since support for pre-macOS 11 has been discontinued makes this easier.

2. IIUC there's no separate implementation for os.Root tooling in Unix and Darwin today. I see that there are plans for similar functionality for other OSes, so a solution would need to accomodate these changes in the future.

3. Thinking where this fast path makes sense; as a hunch, it might not fit well doInRoot. I see it used in multiple places and even though it does handle symlinks that could be short-circuited it also has some caller logic to get right. So maybe I'd start with root.OpenFile -> rootOpenFileNolog and root.OpenRoot -> openRootInRoot.

[gopherbot]

Change https://go.dev/cl/745901 mentions this issue: os: use O_NOFOLLOW_ANY on darwin for improved os.Root performance