Remove Gnus group name encoding/decoding

Remove Gnus group name encoding/decoding

This completes the process started in c1b63af445. Gnus group names are
now fully decoded inside the Gnus system.

* lisp/gnus/gnus-agent.el (gnus-agent-file-coding-system): Change
  default to utf-8-emacs.
  (gnus-agent-decoded-group-names, gnus-agent-decoded-group-name):
  Remove variable and function. Remove all usage in this file.
* lisp/gnus/gnus-cache.el (gnus-cache-decoded-group-names,
  gnus-cache-unified-group-names, gnus-cache-decoded-group-name):
  Remove these variables and function. Remove all usage in this file.
* lisp/gnus/gnus-group.el (gnus-tmp-decoded-group): Remove this
  variable, gnus-tmp-group is now decoded.
  (gnus-group-completing-read): Don't encode or decode group names
  here.
  (gnus-group-make-group): Remove ENCODED argument.
* lisp/gnus/gnus-srvr.el (gnus-browse-foreign-server): Decode group
  names here.
* lisp/gnus/gnus-start.el (gnus-make-hashtable-from-newsrc-alist):
  check for encoded group names and decode.
  (gnus-active-to-gnus-format): Make sure incoming group names are
  decoded.
  (gnus-read-newsrc-el-file): Check for encoded group names in
  gnus-topic-alist.
* lisp/gnus/nnagent.el: Don't use a unibyte buffer.
* lisp/gnus/nnheader.el (nnheader-file-coding-system): Switch default
  from 'raw-text to 'undecided, on the assumption that 'undecided will
  probably write 'utf-8-emacs unless the user has arranged things
  otherwise.
* lisp/gnus/nnimap.el (nnimap-decode-gnus-group,
  nnimap-encode-gnus-group): Remove functions and their use.
* lisp/gnus/nnmail.el (nnmail-parse-active): Remove encoding.
  (nnmail-active-file-coding-system): Default to 'utf-8-emacs instead
  of 'raw-text.
  (nnmail-group-names-not-encoded-p): Obsolete this variable; stop
  using it.
* lisp/gnus/gnus-art.el:
* lisp/gnus/gnus-cus.el:
* lisp/gnus/gnus-msg.el:
* lisp/gnus/gnus-start.el:
* lisp/gnus/gnus-sum.el:
* lisp/gnus/gnus.el:
* lisp/gnus/nnml.el:
* lisp/gnus/message.el:
* lisp/gnus/nnrss.el: Stop using gnus-group-decoded-name in all these
  files.