* mu-register.el (mu-cite-save-registration-file): Use

* mu-register.el (mu-cite-save-registration-file): Use
 `mu-registration-file-coding-system-for-write' for writing a file.
(mu-registration-file-coding-system-for-read): Use `defvar' instead of
 `defcustom' to define it.
(mu-registration-file-coding-system-for-write): New user option.