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.

lisp/gnus/nnagent.el

@@ -117,18 +117,17 @@
   (gnus-request-accept-article "nndraft:queue" nil t t))
 
 (deffoo nnagent-request-set-mark (group action server)
-  (mm-with-unibyte-buffer
-    (insert "(gnus-agent-synchronize-group-flags \""
-	    group
-	    "\" '")
-    (gnus-pp action)
-    (insert " \""
-	    (gnus-method-to-server gnus-command-method)
-	    "\"")
-    (insert ")\n")
-    (let ((coding-system-for-write nnheader-file-coding-system))
-      (write-region (point-min) (point-max) (gnus-agent-lib-file "flags")
-		    t 'silent)))
+  (insert "(gnus-agent-synchronize-group-flags \""
+	  group
+	  "\" '")
+  (gnus-pp action)
+  (insert " \""
+	  (gnus-method-to-server gnus-command-method)
+	  "\"")
+  (insert ")\n")
+  (let ((coding-system-for-write nnheader-file-coding-system))
+    (write-region (point-min) (point-max) (gnus-agent-lib-file "flags")
+		  t 'silent))
   ;; Also set the marks for the original back end that keeps marks in
   ;; the local system.
   (let ((gnus-agent nil))