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.

backend/pcloud/pcloud.go

@@ -671,13 +671,13 @@ func (f *Fs) Copy(ctx context.Context, src fs.Object, remote string) (fs.Object,
 	return dstObj, nil
 }
 
-// Purge deletes all the files and the container
+// Purge deletes all the files in the directory
 //
 // Optional interface: Only implement this if you have a way of
 // deleting all the files quicker than just running Remove() on the
 // result of List()
-func (f *Fs) Purge(ctx context.Context) error {
-	return f.purgeCheck(ctx, "", false)
+func (f *Fs) Purge(ctx context.Context, dir string) error {
+	return f.purgeCheck(ctx, dir, false)
 }
 
 // CleanUp empties the trash