Apply the elements of `file-local-variables-alist'.
If there are any elements, runs `before-hack-local-variables-hook',
then calls `hack-one-local-variable' to apply the alist elements one by one.
Finishes by running `hack-local-variables-hook', regardless of whether
the alist is empty or not.
Note that this function ignores a `mode' entry if it specifies the same
major mode as the buffer already has.