Fix encoding and access of Gnus group names

Fix encoding and access of Gnus group names

* lisp/gnus/gnus-start.el (gnus-active-to-gnus-format): Encode group
  names as 'latin-1.
* lisp/gnus/nnmail.el (nnmail-parse-active): Ditto.
* lisp/gnus/nnml.el (nnml-request-group, nnml-request-create-group,
  nnml-request-expire-articles, nnml-request-delete-group,
  nnml-request-rename-group, nnml-deletable-article-p,
  nnml-active-number, nnml-open-incremental-nov): Use assoc-string
  with nnml-group-alist.
* lisp/gnus/nnrss.el (nnrss-request-delete-group,
  nnrss-retrieve-groups, nnrss-read-group-data, nnrss-check-group,
  nnrss-generate-download-script): Use assoc-string with
  nnrss-group-alist.