(Finsert_file_contents): Setup src_multibyte and

(Finsert_file_contents): Setup src_multibyte and
dst_multibyte members of coding.  On handling REPLACE on unibyte
buffer, convert the result of decode_coding to unibyte.  On
inserting into a mutibyte buffer, always call code_convert_region.
(e_write): Setup cdoing->src_multibyte according to the
multibyteness of the source (buffer or string).