Desktop: protect users against inadvertant upgrading of desktop file.

Desktop: protect users against inadvertant upgrading of desktop file.

An upgraded (version 208) desktop file cannot be read in Emacs < 25.

* etc/NEWS: Add an entry about upgrading a desktop file.

* lisp/desktop.el (desktop-file-version): Amend doc string.
(desktop-native-file-version, desktop-io-file-version): new variables.
(desktop-clear): Set desktop-io-file-version to nil.
(desktop-buffer-info): make the presence of the last item on the list
conditional on (>= desktop-io-file-version 208).
(desktop-save): Add extra parameter VERSION to take user's C-u or C-u C-u.
Amend the doc string.  Add code to determine the output file version.
(desktop-create-buffer): Set desktop-io-file-version to the input file's
version.