fs: Add directory to optional Purge interface - fixes #1891

- add a directory to the optional Purge interface
- fix up all the backends
- add an additional integration test to test for the feature
- use the new feature in operations.Purge

Many of the backends had been prepared in advance for this so the
change was trivial for them.

fs/fs_test.go

@@ -49,7 +49,7 @@ func TestFeaturesList(t *testing.T) {
 func TestFeaturesEnabled(t *testing.T) {
 	ft := new(Features)
 	ft.CaseInsensitive = true
-	ft.Purge = func(ctx context.Context) error { return nil }
+	ft.Purge = func(ctx context.Context, dir string) error { return nil }
 	enabled := ft.Enabled()
 
 	flag, ok := enabled["CaseInsensitive"]