Fix bug #13262 with crashes in completion on MS-Windows with non-ASCII filenames.

Fix bug #13262 with crashes in completion on MS-Windows with non-ASCII filenames.

 src/fileio.c (file_name_as_directory, directory_file_name): Accept
 an additional argument MULTIBYTE to indicate whether the input C
 came from a multibyte or a unibyte Lisp string; all callers
 adjusted.  Don't assume the input string is always multibyte.
 (Ffile_name_directory) [DOS_NT]: Handle unibyte strings correctly:
 don't ENCODE_FILE them, and return a unibyte string if the input
 was unibyte.
 (Fexpand_file_name): Don't mix unibyte with multibyte strings, and
 don't assume the input strings will always be multibyte.  If the
 input strings are multibyte, decode strings obtained from C
 library functions.