(Fadd_text_properties): Don't treat the initial

(Fadd_text_properties): Don't treat the initial
interval specially, aside from splitting off the part starting
where we want to start changing things.
(Fremove_text_properties): Likewise.
For ending interval, split it left as in Fadd_text_properties.
(Fset_text_properties): Don't set starting interval if it goes
past END.  Instead, split it left and set the left part.