(load_charset_map): Set the default value of encoder

(load_charset_map): Set the default value of encoder
and deunifier char-tables to nil.
(map_charset_chars): Argument changed.  Callers changed.  Use
map_char_table_for_charset instead of map_char_table.
(Fmap_charset_chars): New optional args from_code and to_code.
(Fdefine_charset_internal): Adjusted for the change of
`define-charset' (:parents -> :subset or :superset).
(charset_work): New variable.
(encode_char): Adjusted for the change of
Fdefine_charset_internal.
(syms_of_charset): Likewise.