(read-directory-name): Always pass non-nil

(read-directory-name): Always pass non-nil
DEFAULT-FILENAME arg to read-file-name.
(backup-buffer-copy, basic-save-buffer-2): Take care against
writing thru an unexpected existing symlink.
(revert-buffer): In indirect buffer, revert the base buffer.
(magic-mode-alist): Doc fix.
(buffer-stale-function): Doc fix.
(minibuffer-with-setup-hook): Avoid warning.
(mode-require-final-newline): Doc and custom fix.