(READ_QUADRUPLET_BYTE): Set *NCHARS_RETURN before

(READ_QUADRUPLET_BYTE): Set *NCHARS_RETURN before
returning.
(base64_encode_1): Make it work for a text of multibyte form.
(Fbase64_decode_region): Allocate sufficient memory for multibyte
case.  Don't call str_to_multibyte because base64_decode_1
produces correct multibyte form for eight-bit codes.
(Fbase64_decode_string): Adjusted for the change of
base64_decode_1.
(base64_decode_1): New args MULTIBYTE and NCHARS_RETURN.  If
MULTIBYTE is nonzero, produce correct multibyte form for eight-bit
codes.