Read a character set from the minibuffer, prompting with string PROMPT.
It must be an Emacs character set listed in the variable `charset-list'.
Optional arguments are DEFAULT-VALUE and INITIAL-INPUT.
DEFAULT-VALUE, if non-nil, is the default value.
INITIAL-INPUT, if non-nil, is a string inserted in the minibuffer initially.
See the documentation of the function `completing-read' for the detailed
meanings of these arguments.
(fn PROMPT &optional DEFAULT-VALUE INITIAL-INPUT)