(Fdo_auto_save): Save and restore multibyteness of minibuffer.

(Fdo_auto_save): Save and restore multibyteness of minibuffer.
(Fwrite_region): Use message_with_string.
(auto_save_error, Fdo_auto_save): Likewise.
(Ffile_name_directory): Maybe use make_unibyte_string.
(Ffile_name_nondirectory): Likewise.
(Fsubstitute_in_file_name): Convert substituted envvars to multibyte
if necessary.  Otherwise use make_unibyte_string.
(Finsert_file_contents): Use make_unibyte_string.
(double_dollars): Use make_uninit_multibyte_string.