(char_quoted): Use FETCH_CHAR_AS_MULTIBYTE to convert

(char_quoted): Use FETCH_CHAR_AS_MULTIBYTE to convert
unibyte chars to multibyte.
(back_comment): Likewise.
(scan_words): Likewise.
(skip_chars): The arg syntaxp is deleted, and the code for
handling syntaxes is moved to skip_syntaxes.  Callers changed.
Fix the case that the multibyteness of STRING and the current
buffer doesn't match.
(skip_syntaxes): New function.
(SYNTAX_WITH_MULTIBYTE_CHECK): Check C by ASCII_CHAR_P, not by
SINGLE_BYTE_CHAR_P.
(Fforward_comment): Use FETCH_CHAR_AS_MULTIBYTE to convert unibyte
chars to multibyte.
(scan_lists): Likewise.
(Fbackward_prefix_chars): Likewise.
(scan_sexps_forward): Likewise.