accounting: Make checkers show what they are doing

Before this change, all types of checkers showed "checking" after the
file name despite the fact that not all of them were checking.

After this change, they can show

- checking
- deleting
- hashing
- importing
- listing
- merging
- moving
- renaming

See: https://forum.rclone.org/t/what-is-rclone-checking-during-a-purge/35931/

backend/b2/b2.go

@@ -1221,7 +1221,7 @@ func (f *Fs) purge(ctx context.Context, dir string, oldOnly bool) error {
 					fs.Errorf(object.Name, "Can't create object %v", err)
 					continue
 				}
-				tr := accounting.Stats(ctx).NewCheckingTransfer(oi)
+				tr := accounting.Stats(ctx).NewCheckingTransfer(oi, "deleting")
 				err = f.deleteByID(ctx, object.ID, object.Name)
 				checkErr(err)
 				tr.Done(ctx, err)
@@ -1235,7 +1235,7 @@ func (f *Fs) purge(ctx context.Context, dir string, oldOnly bool) error {
 			if err != nil {
 				fs.Errorf(object, "Can't create object %+v", err)
 			}
-			tr := accounting.Stats(ctx).NewCheckingTransfer(oi)
+			tr := accounting.Stats(ctx).NewCheckingTransfer(oi, "checking")
 			if oldOnly && last != remote {
 				// Check current version of the file
 				if object.Action == "hide" {