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/s3/s3.go

@@ -4542,7 +4542,7 @@ func (f *Fs) purge(ctx context.Context, dir string, oldOnly bool) error {
 			fs.Errorf(object, "Can't create object %+v", err)
 			return nil
 		}
-		tr := accounting.Stats(ctx).NewCheckingTransfer(oi)
+		tr := accounting.Stats(ctx).NewCheckingTransfer(oi, "checking")
 		// Work out whether the file is the current version or not
 		isCurrentVersion := !versioned || !version.Match(remote)
 		fs.Debugf(nil, "%q version %v", remote, version.Match(remote))