fs: create Overlay feature flag to indicate backend wraps others

Set this automatically for any backend which implements UnWrap and
manually for combine and union which can't implement UnWrap but do
overlay other backends.

backend/combine/combine.go

@@ -299,6 +299,9 @@ func NewFs(ctx context.Context, name, root string, m configmap.Mapper) (outFs fs
 		}
 	}
 
+	// show that we wrap other backends
+	features.Overlay = true
+
 	f.features = features
 
 	// Get common intersection of hashes