(struct decoding_stream): Rename member `CH' to `CPOS'.

(struct decoding_stream): Rename member `CH' to `CPOS'.
(reset_decoding_stream): Use `str->cpos' instead of `str->ch'.
(decode_coding_sjis): Likewise.
(decode_coding_big5): Likewise.
(decode_coding_ucs4): Likewise.
(decode_coding_utf8): Likewise.
(parse_iso2022_esc): Don't use `CHARSET_TYPE_*'; modify for
`CHARSET_BY_ATTRIBUTES'.
(decode_coding_iso2022): Use `str->cpos' instead of `str->ch'; use
`str->counter'; decode 3, 4 bytes sets.
(char_encode_iso2022): Don't use `BREAKUP_CHAR'; encode 3, 4 bytes
sets.
(decode_coding_no_conversion): Use `str->cpos' instead of `str->ch'.