Use Enriched mode in etc/HELLO to keep charset information

Use Enriched mode in etc/HELLO to keep charset information

This allows to encode HELLO in UTF-8, thus supporting the entire
repertory of Unicode, while still keeping the charset info where
that is important.  Suggested by Michael Welsh Duggan <mwd@md5i.com>.
* lisp/textmodes/enriched.el (enriched-translations): Add
translations for 'charset'.
(enriched-decode-charset, enriched-handle-charset-prop): New
functions.
* lisp/facemenu.el (facemenu-special-menu): Add sub-menu for
'charset' property.
(facemenu-set-charset): New function.
(facemenu-remove-special): Remove the 'charset' property as well.

* etc/NEWS: Announce the new feature of Enriched mode.
* etc/HELLO: Recode in UTF-8 and place under Enriched mode.

* doc/emacs/text.texi (Enriched Properties): Mention the support
for 'charset'.