bug#12989: 24.3.50; buffer-file-type is not buffer-local

[Kazuhiro Ito <kzhr@d1.dion.ne.jp>]

When I evaluate below code, Emacs-23 and trunk on Windows return the
different results.

(list
 (setq buffer-file-type nil)
 (progn
   (let ((coding-system-for-read 'no-conversion))
     ;; Specify non-existing file.
     (kill-buffer (find-file-noselect "c:/zzzzzzz")))
   buffer-file-type))

-> (nil nil) (Emacs 23)
-> (nil t) (trunk)

And, if buffer-file-type is set to t, many file coding system
detections fail.  Docstring says that buffer-file-type autmatically
becomes buffer-local, but that is not true on trunk.

> buffer-file-type is a variable defined in `subr.el'.
> Its value is nil
> 
> Documentation:
> Non-nil if the visited file is a binary file.
> This variable is meaningful on MS-DOG and Windows NT.
> On those systems, it is automatically local in every buffer.
> On other systems, this variable is normally always nil.


Additionally, there is typo in docsstring of buffer-file-type.

> This variable is meaningful on MS-DOG and Windows NT.
                                 ~~~~~~

-- 
Kazuhiro Ito