(list-character-sets-1): New subroutine.

(list-character-sets-1): New subroutine.
(list-character-sets): Use it.
(list-coding-systems-1): New subroutine.
(list-coding-systems): Use it.
(list-input-methods-1): New subroutine.
(list-input-methods): Use it.
(mule-diag): Avoid method of displaying text in *Help* then copying it.
Instead, insert it directly into *Mule-Diagnosis*.
Use list-character-sets-1, list-coding-systems-1, list-input-methods-1.
Copy the code from list-fontsets and list-coding-categories.
Improve the display buffer's header.