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.

lisp/calendar/todo-mode.el

@@ -4726,9 +4726,8 @@ name in `todo-directory'.  See also the documentation string of
 	      (todo-convert-legacy-date-time)))
 	    (forward-line))
 	  (setq file (concat todo-directory
-			     (read-string
-			      (format "Save file as (default \"%s\"): " default)
-			      nil nil default)
+			     (read-string (format-prompt "Save file as" default)
+			                  nil nil default)
 			     ".todo"))
 	  (unless (file-exists-p todo-directory)
 	    (make-directory todo-directory))

lisp/cmuscheme.el

@@ -327,9 +327,8 @@ With a prefix argument switch off tracing of procedure PROC."
   (interactive
    (list (let ((current (symbol-at-point))
                (action (if current-prefix-arg "Untrace" "Trace")))
-           (if current
-               (read-string (format "%s procedure [%s]: " action current) nil nil (symbol-name current))
-             (read-string (format "%s procedure: " action))))
+           (read-string (format-prompt "%s procedure" current action)
+                        nil nil (and current (symbol-name current))))
          current-prefix-arg))
   (when (= (length proc) 0)
     (error "Invalid procedure name"))

lisp/printing.el

@@ -5284,22 +5284,18 @@ If menu binding was not done, calls `pr-menu-bind'."
 
 
 (defun pr-interactive-n-up (mess)
-  (or (stringp mess) (setq mess "*"))
-  (save-match-data
-    (let* ((fmt-prompt "%s[%s] N-up printing (default 1): ")
-	   (prompt "")
-	   (str (read-string (format fmt-prompt prompt mess) nil nil "1"))
-	   int)
-      (while (if (string-match "^\\s *[0-9]+$" str)
-		 (setq int (string-to-number str)
-		       prompt (cond ((< int 1)   "Integer below 1; ")
-				    ((> int 100) "Integer above 100; ")
-				    (t           nil)))
-	       (setq prompt "Invalid integer syntax; "))
-	(ding)
-	(setq str
-	      (read-string (format fmt-prompt prompt mess) str nil "1")))
-      int)))
+  (unless (stringp mess)
+    (setq mess "*"))
+  (let (int)
+    (while (or (< (setq int (read-number (format "[%s] N-up printing:" mess) 1))
+                  0)
+               (> int 100))
+      (if (< int 0)
+	  (message "Integer below 1")
+	(message "Integer above 100"))
+      (sit-for 1)
+      (ding))
+    int))
 
 
 (defun pr-interactive-dir (mess)
@@ -5323,7 +5319,7 @@ If menu binding was not done, calls `pr-menu-bind'."
 
 
 (defun pr-interactive-regexp (mess)
-  (read-string (format "[%s] File regexp to print: " mess) nil nil ""))
+  (read-string (format "[%s] File regexp to print: " mess)))
 
 
 (defun pr-interactive-dir-args (mess)

lisp/progmodes/prolog.el

@@ -2369,12 +2369,8 @@ In effect it sets the `fill-prefix' when inside comments and then calls
    ;; in prolog-help-function-i
    (t
     (let* ((word (prolog-atom-under-point))
-           (predicate (read-string
-                       (format "Help on predicate%s: "
-                               (if word
-                                   (concat " (default " word ")")
-                                 ""))
-                       nil nil word))
+           (predicate (read-string (format-prompt "Help on predicate" word)
+                                   nil nil word))
            ;;point
            )
       (if prolog-help-function-i