Comment 3 for bug 776532

I don't see any reason we need to break the API here. Anyone calling nih_dir_walk() directly should already be using values of data, filter, and visitor that are compatible. The only problem here is that nih_watch_add() is calling them in a manner that requires two different types for 'data'. So this is a bug in the caller; if other callers have this same bug, that's a bug in those other callers. This is not something we need (or want) to solve generically in nih_dir_walk() itself.