(autoload-find-destination): Return nil

(autoload-find-destination): Return nil
rather than throwing `up-to-date'.
(autoload-generate-file-autoloads): Adjust correspondingly.
(update-file-autoloads): Be careful to let-bind
autoload-modified-buffers and adjust to new calling conventions.
(autoload-modified-buffers): Make it a dynamically scoped var.
(update-directory-autoloads): Use file-relative-name instead of
autoload-trim-file-name.
(autoload-insert-section-header): Don't use autoload-trim-file-name
since the file is already relative now.
(autoload-trim-file-name): Remove.