Accept new always' value for option buffer-offer-save'

Also revert ee512e9a82

* lisp/files.el (buffer-offer-save): In addition to nil and t, now
  allows a third symbol value, `always'. A buffer where this option is
  set to `always' will always be offered for save by
  `save-some-buffers'.
  (save-some-buffers): Check the exact value of this buffer-local
  variable. No longer check the buffer name, or the value of
  `write-contents-functions'.
* doc/lispref/buffers.texi (Killing Buffers): Note change in manual.
* doc/lispref/files.texi (Saving Buffers): Remove note about buffer
  names.
* etc/NEWS: Mention in NEWS.

doc/lispref/buffers.texi

@@ -1089,12 +1089,15 @@ is not cleared by changing major modes.
 
 @defopt buffer-offer-save
 This variable, if non-@code{nil} in a particular buffer, tells
-@code{save-buffers-kill-emacs} and @code{save-some-buffers} (if the
-second optional argument to that function is @code{t}) to offer to
-save that buffer, just as they offer to save file-visiting buffers.
-@xref{Definition of save-some-buffers}.  The variable
-@code{buffer-offer-save} automatically becomes buffer-local when set
-for any reason.  @xref{Buffer-Local Variables}.
+@code{save-buffers-kill-emacs} to offer to save that buffer, just as
+it offers to save file-visiting buffers.  If @code{save-some-buffers}
+is called with the second optional argument set to @code{t}, it will
+also offer to save the buffer.  Lastly, if this variable is set to the
+symbol @code{always}, both @code{save-buffers-kill-emacs} and
+@code{save-some-buffers} will always offer to save.  @xref{Definition
+of save-some-buffers}.  The variable @code{buffer-offer-save}
+automatically becomes buffer-local when set for any reason.
+@xref{Buffer-Local Variables}.
 @end defopt
 
 @defvar buffer-save-without-query

doc/lispref/files.texi

@@ -368,8 +368,7 @@ With an argument of 0, unconditionally do @emph{not} make any backup file.
 This command saves some modified file-visiting buffers.  Normally it
 asks the user about each buffer.  But if @var{save-silently-p} is
 non-@code{nil}, it saves all the file-visiting buffers without
-querying the user.  Additionally, buffers whose name begins with a
-space (``internal'' buffers) will not be offered for save.
+querying the user.
 
 @vindex save-some-buffers-default-predicate
 The optional @var{pred} argument provides a predicate that controls

etc/NEWS

@@ -117,6 +117,11 @@ The effect is similar to that of "toolBar" resource on the tool bar.
 
 * Changes in Emacs 26.1
 
++++
+** Option 'buffer-offer-save' can be set to new value, 'always'.  When
+   set to 'always', the command `save-some-buffers' will always offer
+   this buffer for saving.
+
 ** Security vulnerability related to Enriched Text mode is removed.
 
 +++
@@ -144,8 +149,7 @@ init file:
 'save-buffer' process.  Previously, saving a buffer that was not
 visiting a file would always prompt for a file name.  Now it only does
 so if 'write-contents-functions' is nil (or all its functions return
-nil).  A non-nil buffer-local value for this variable is sufficient
-for 'save-some-buffers' to consider the buffer for saving.
+nil).
 
 ---
 ** New variable 'executable-prefix-env' for inserting magic signatures.

lisp/files.el

@@ -150,8 +150,13 @@ Called with an absolute file name as argument, it returns t to enable backup.")
 (defcustom buffer-offer-save nil
   "Non-nil in a buffer means always offer to save buffer on exit.
 Do so even if the buffer is not visiting a file.
-Automatically local in all buffers."
-  :type 'boolean
+Automatically local in all buffers.
+
+Set to the symbol `always' to offer to save buffer whenever
+`save-some-buffers' is called."
+  :type '(choice (const :tag "Never" nil)
+                 (const :tag "On Emacs exit" t)
+                 (const :tag "Whenever save-some-buffers is called" always))
   :group 'backup)
 (make-variable-buffer-local 'buffer-offer-save)
 (put 'buffer-offer-save 'permanent-local t)
@@ -5188,15 +5193,11 @@ change the additional actions you can take on files."
 	       (and (buffer-live-p buffer)
 		    (buffer-modified-p buffer)
                     (not (buffer-base-buffer buffer))
-                    (not (eq (aref (buffer-name buffer) 0) ?\s))
                     (or
                      (buffer-file-name buffer)
-                     (and pred
-                          (progn
-                            (set-buffer buffer)
-                            (and buffer-offer-save (> (buffer-size) 0))))
-                     (buffer-local-value
-                      'write-contents-functions buffer))
+                     (with-current-buffer buffer
+                       (or (eq buffer-offer-save 'always)
+                           (and pred buffer-offer-save (> (buffer-size) 0)))))
                     (or (not (functionp pred))
                         (with-current-buffer buffer (funcall pred)))
                     (if arg