Function: mm-enrich-utf-8-by-mule-ucs

Make the `utf-8' MIME charset usable by the Mule-UCS package.
This function will run when the `un-define' module is loaded under
XEmacs, and fill the `utf-8' entry in `mm-mime-mule-charset-alist'
with Mule charsets. It is completely useless for Emacs.