(backup, find-file): Make `files' their parent.

(backup, find-file): Make `files' their parent.

(view-read-only): New option variable.  If
non-nil then buffers visiting files read-only, do it in view mode.
(find-file-read-only, find-file-read-only-other-window)
(find-file-read-only-other-frame): Call toggle-read-only
instead of setting buffer-read-only explicitly.
(toggle-read-only, after-find-file): Handle view-read-only.
(save-some-buffers): Use second argument of view-buffer
instead of setting view-exit-action.