Replace UserIDRange/GroupIDRange by IDRange in internal type to reduce difference with external type.

We had IDRange in both types prior 9440a687 commit that splitted it
into UserIDRange/GroupIDRange. Later, in c91a12d2 commit we had to
revert this changes because they broke backward compatibility but
UserIDRange/GroupIDRange struct left in the internal type.

This commit removes these leftovers and reduces the differences
between internal and external types.