Define and use new procedure display-symbol-keys-p

* lisp/frame.el (display-symbol-keys-p): Define.

* lisp/simple.el (normal-erase-is-backspace-setup-frame): Use eq
instead of memq.
(normal-erase-is-backspace-mode): Use display-symbol-keys-p.

etc/NEWS

@@ -1230,6 +1230,12 @@ the 128...255 range, as expected.
 This allows to create and parent immediately a minibuffer-only child
 frame when making a frame.
 
+---
+*** New predicates 'display-blink-cursor-p' and 'display-symbol-keys-p'.
+These predicates are to be preferred over 'display-graphic-p' when
+testing for blinking cursor capability and the capability to have
+symbols (e.g., [return], [tab], [backspace]) as keys respectively.
+
 ** Tabulated List mode
 
 +++

lisp/frame.el

@@ -1927,6 +1927,16 @@ frame's display)."
      (t
       nil))))
 
+(defun display-symbol-keys-p (&optional display)
+  "Return non-nil if DISPLAY supports symbol names as keys.
+This means that, for example, DISPLAY can differentiate between
+the keybinding RET and [return]."
+  (let ((frame-type (framep-on-display display)))
+    (or (memq frame-type '(x w32 ns pc))
+        ;; MS-DOS and MS-Windows terminals have built-in support for
+        ;; function (symbol) keys
+        (memq system-type '(ms-dos windows-nt)))))
+
 (declare-function x-display-screens "xfns.c" (&optional terminal))
 
 (defun display-screens (&optional display)

lisp/simple.el

@@ -8690,7 +8690,7 @@ call `normal-erase-is-backspace-mode' (which see) instead."
                (and (not noninteractive)
                     (or (memq system-type '(ms-dos windows-nt))
 			(memq window-system '(w32 ns))
-                        (and (memq window-system '(x))
+                        (and (eq window-system 'x)
                              (fboundp 'x-backspace-delete-keys-p)
                              (x-backspace-delete-keys-p))
                         ;; If the terminal Emacs is running on has erase char
@@ -8701,6 +8701,8 @@ call `normal-erase-is-backspace-mode' (which see) instead."
              normal-erase-is-backspace)
            1 0)))))
 
+(declare-function display-symbol-keys-p "frame" (&optional display))
+
 (define-minor-mode normal-erase-is-backspace-mode
   "Toggle the Erase and Delete mode of the Backspace and Delete keys.
 
@@ -8736,8 +8738,7 @@ See also `normal-erase-is-backspace'."
   (let ((enabled (eq 1 (terminal-parameter
                         nil 'normal-erase-is-backspace))))
 
-    (cond ((or (memq window-system '(x w32 ns pc))
-	       (memq system-type '(ms-dos windows-nt)))
+    (cond ((display-symbol-keys-p)
 	   (let ((bindings
 		  '(([M-delete] [M-backspace])
 		    ([C-M-delete] [C-M-backspace])