Use format-prompt in some read-string calls

Use format-prompt in some read-string calls

* lisp/progmodes/prolog.el (prolog-help-on-predicate):
* lisp/cmuscheme.el (scheme-trace-procedure):
* lisp/calendar/todo-mode.el (todo-convert-legacy-files): Use
format-prompt in some read-string calls (that have default values).

* lisp/printing.el (pr-interactive-regexp): No need to use "" as
the default value, because that's the default default value.
(pr-interactive-n-up): Use read-number instead of read-string and
then parsing the string.