* bookmark.el: Provide a generic exit hook, as suggested by Ovidiu

* bookmark.el: Provide a generic exit hook, as suggested by Ovidiu
Predescu <ovidiu@cup.hp.com>:
(bookmark-exit-hook): new var.
(bookmark-exit-hook-internal): new func, replaces old raw lambda form
in `kill-emacs-hook', and runs new `bookmark-exit-hooks'.  No longer
tests for the bookmark feature, as logically that feature must have
been provided if this function is running.
Removed ;;;###autoload before the `add-hook' call.