Remove path flags from `link-at` ?

[wingo]

Currently, link-at looks like this:

// link-at: async func(old-path-flags: path-flags, old-path: string,
                       new-descriptor: borrow<descriptor>, new-path: string)
                       -> result<_, error-code>;

The old-path-flags can either be empty or symlink-follow. Thing is, wasmtime does not support symlink-follow on link-at, returning an error immediately. Perhaps this is reasonable, as you can always racily readlink the old-path yourself, and the plain link syscall does not dereference symbolic links.

So, should we keep the path flags, or is wasmtime in the wrong for not allowing for symlink-follow ? Cc @sunfishcode, @alexcrichton.

[pchickey]

If there is no reasonable way to implement the symlink-follow flag, then we should take this opportunity to get rid of the flag!