Define mouse-set-font in mule-menu-keymap.

Disable menu set-buffer-process-coding-system if there is no
process for the current buffer.
(command-execute-with-coding-system): New function.
(universal-coding-system-argument): Make it handle
universal-arguement correctly.
(describe-language-support): Call coding-system-doc-string instead
of coding-system-docstring.

lisp/international/mule-cmds.el

@@ -561,7 +561,7 @@ and sometimes other things."
 	    (princ (format "  %s (`%c' in mode line):\n\t%s\n"
 			   (car l)
 			   (coding-system-mnemonic (car l))
-			   (coding-system-docstring (car l))))
+			   (coding-system-doc-string (car l))))
 	    (setq l (cdr l))))))))
 
 ;;; Charset property