os: os.Root: use OBJ_DONT_REPARSE on Windows

[neild]

Windows's NtCreateFile takes an OBJECT_ATTRIBUTES struct indicating the file to open. Setting the OBJ_DONT_REPARSE flag in this struct disables following symlinks (and other forms of reparse point). We can use this to implement a fast path in the common case when the target of an os.Root operation does not contain any reparse points.

[gopherbot]

Change https://go.dev/cl/674675 mentions this issue: os: use OBJ_DONT_REPARSE for Root operations on Windows