(SPLIT_COMPOSITE_SEQ): New macro.

(SPLIT_COMPOSITE_SEQ): New macro.
(SPLIT_CHARACTER_SEQ): New macro.
(SPLIT_MULTIBYTE_SEQ): New macro.
(CHAR_COMPONENT_VALID_P): New macro.
(non_ascii_char_to_string): Generate a multibyte sequence as far
as possible.
(string_to_non_ascii_char): The 4th arg exclude_tail_garbage is
deleted.  Caller changed.  Use the macro SPLIT_MULTIBYTE_SEQ.
(split_non_ascii_string): Likewise.
(multibyte_form_length): Use the macro PARSE_MULTIBYTE_SEQ.
(char_printable_p): New function.
(translate_char): Check character by NATNUMP instead of INTEGERP.
(unibyte_char_to_multibyte): Call char_valid_p instead of
VALID_MULTIBYTE_CHAR_P.
(Fmake_char_internal): Check the arguments more rigidly.
(Fcharset_after): Use the macro SPLIT_MULTIBYTE_SEQ.
(char_valid_p): Check the validity by CHAR_COMPONENT_VALID_P.
(Fmultibyte_char_to_unibyte): Check the validity of character by
CHAR_VALID_P.
(chars_in_text): Call multibyte_chars_in_text.
(multibyte_chars_in_text): Use the macro PARSE_MULTIBYTE_SEQ.
(Fcompose_string): Use the macro STRING_CHAR_AND_LENGTH instead of
STRING_CHAR_AND_CHAR_LENGTH (which is obsolete now).