Make ido-mode override ffap-file-finder

* lisp/ffap.el: Autoload so that we can override in ido (bug#21980).

* lisp/ido.el (ido-everywhere): Override ffap-file-finder.

etc/NEWS

@@ -2410,12 +2410,6 @@ leak information from the reporting user.
 *** 'count-windows' now takes an optional parameter ALL-FRAMES.
 The semantics are as with 'walk-windows'.
 
----
-*** Killing virtual ido buffers interactively will make them go away.
-Previously, killing a virtual ido buffer with 'ido-kill-buffer' didn't
-do anything.  This has now been changed, and killing virtual buffers
-with that command will remove the buffer from recentf.
-
 ---
 *** New variable 'ffap-file-name-with-spaces'.
 If non-nil, 'find-file-at-point' and friends will try to guess more
@@ -2490,6 +2484,17 @@ height of lines or width of chars.
 When non-nil, use a new xwidget webkit session after bookmark jump.
 Otherwise, it will use 'xwidget-webkit-last-session'.
 
+** ido
+
+---
+*** Switching on 'ido-mode' now also overrides 'ffap-file-finder'.
+
+---
+*** Killing virtual ido buffers interactively will make them go away.
+Previously, killing a virtual ido buffer with 'ido-kill-buffer' didn't
+do anything.  This has now been changed, and killing virtual buffers
+with that command will remove the buffer from recentf.
+
 ** Flymake mode
 
 +++

lisp/ffap.el

@@ -260,6 +260,7 @@ ffap most of the time."
   :type 'boolean
   :group 'ffap)
 
+;;;###autoload
 (defcustom ffap-file-finder 'find-file
   "The command called by `find-file-at-point' to find a file."
   :type 'function

lisp/ido.el

@@ -1521,6 +1521,10 @@ Removes badly formatted data and ignored directories."
   :global t
   (remove-function read-file-name-function #'ido-read-file-name)
   (remove-function read-buffer-function #'ido-read-buffer)
+  (when (boundp 'ffap-file-finder)
+    (remove-function ffap-file-finder #'ido-find-file)
+    (when ido-mode
+      (add-function :override ffap-file-finder #'ido-find-file)))
   (when ido-everywhere
     (if (not ido-mode)
         (ido-mode 'both)