lisp/desktop.el: Make some frame-restoring functions public.

lisp/desktop.el: Make some frame-restoring functions public.
(desktop-saved-frame-states): Rename from desktop--saved-states;
all users changed.
(desktop-save-frames): Rename from desktop--save-frames.
Do not save state to desktop file.
(desktop-save): Save desktop-saved-frame-states to desktop file
and reset to nil.
(desktop-restoring-frames-p): New function.
(desktop-restore-frames): Use it.  Rename from desktop--restore-frames.
(desktop-read): Use desktop-restoring-frames-p.  Do not try to fix
buffer-lists when restoring frames.  Suggested by Martin Rudalics.