(list-character-sets): Set major mode of *Help*

(list-character-sets): Set major mode of *Help*
buffer to help-mode.
(describe-coding-system): If user input null for coding system,
call describe-current-coding-system.
(describe-current-coding-system-briefly): Doc-string modified.
(print-coding-system-briefly): Print parent and alises of coding
system.
(describe-current-coding-system): Show more information neatly.
(list-coding-systems): If called interactively, do not list up
coding categories.
(list-input-methods): New function.
(mule-diag): Call list-input-methods for listing input methods.