(autoload-ensure-default-file, autoload-insert-section-header)

(autoload-ensure-default-file, autoload-insert-section-header)
(autoload-before-p, autoload-remove-section): New functions.
(generate-file-autoloads): Use them.
(update-file-autoloads): Use them.  Return FILE if there's no cookie.
Make sure the autoload file is properly formatted when creating it.
(update-autoloads-from-directories): Use them as well.
Only update autoloads for files whose timestamp has changed.
Add a dummy entry to remember the files which had no autoload cookies.