Define describe-coding-system instead of

Define describe-coding-system instead of
describe-current-coding-system in help-map and mule-menu-keymap.
In mule-menu-keymap, enable set-buffer-process-coding-system only
when the current buffer has been associated with some subprocess.
(set-language-info): Doc-string modified because input-method is
not a valid KEY argument now.
(leim-list-file-name, leim-list-header, leim-list-entry-regexp):
New variables.
(update-leim-list-file, update-all-leim-list-files): New
functions.
(current-input-method): Doc-string modified because the value is
now input method name.
(defualt-input-method, previous-input-method): Likewise.
(current-input-method-title): Doc-string modified because now each
input method does not have to set this variable.
(input-method-alist): New variable.
(register-input-method): Register input method in
input-method-alist.
(read-language-and-input-method-name): Deleted.
(read-input-method-name): New function.
(activate-input-method, select-input-method, toggle-input-method):
Modified for the above change.
(read-multilingual-string): Likewise.
(describe-current-input-method): Renamed from
describe-input-method.
(describe-input-method): New function.
(describe-language-environment): Do not put a vacant line at the
top of *Help* buffer.  Show a title string of each input method.