Temporarily preserve encoded Gnus group names in Gnus files

Temporarily preserve encoded Gnus group names in Gnus files

Non-ascii Gnus groups should be written to files in their encoded
version until we're ready to bump Gnus' version and add an upgrade
routine.

* lisp/gnus/gnus-start.el (gnus-gnus-to-quick-newsrc-format):
* lisp/gnus/gnus-agent.el (gnus-category-read):
  (gnus-category-write): Handle non-ascii group names appropriately.
* lisp/gnus/gnus-registry.el (gnus-registry--munge-group-names): New
  function to encode/decode group names.
  (gnus-registry-fixup-registry):
  (gnus-registry-save): Use function.