Remove path flags from `link-at` ?

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.

Start by reviewing the link-at signature and the stated behavior of wasmtime for symlink-follow. Determine whether the path flags should be removed from the interface or whether wasmtime should support them; done means resolving that specification question and identifying the required behavior.