(MAKE_NON_ASCII_CHAR): Handle the case that C1 or C2
are negative.
(MAKE_CHAR): Don't set MSBs of C1 and C2 to 0.
(VALID_MULTIBYTE_CHAR_P): This macro deleted.
(PARSE_COMPOSITE_SEQ): New macro.
(PARSE_CHARACTER_SEQ): New macro.
(PARSE_MULTIBYTE_SEQ): New macro.
(CHAR_PRINTABLE_P): New macro.
(STRING_CHAR): Adjusted for the change of string_to_non_ascii_char.
(STRING_CHAR_AND_LENGTH): Likewise.
(STRING_CHAR_AND_CHAR_LENGTH): Define it as STRING_CHAR_AND_LENGTH.
(INC_POS): Use the macro PARSE_MULTIBYTE_SEQ.
(DEC_POS, BUF_INC_POS, BUF_DEC_POS): Likewise,