Handle the new convention that `position' values

Handle the new convention that `position' values
in a string's intervals start from zero.
(validate_interval_range, interval_of, Fnext_property_change)
(property_change_between_p, Fnext_single_property_change)
(Fprevious_property_change, Fprevious_single_property_change):
(Ftext_property_any, Ftext_property_not_all): Implement that.

Undo previous change.