Avoid encoding file names run through dostounix_filename on MS-Windows.

Avoid encoding file names run through dostounix_filename on MS-Windows.

 src/w32.c (normalize_filename): Accept an additional argument
 MULTIBYTE; if non-zero, traverse the file name by bytes and don't
 downcase it even if w32-downcase-file-names is non-nil.
 (dostounix_filename): Accept an additional argument MULTIBYTE and
 pass it to normalize_filename.
 (emacs_root_dir): Adjust.
 src/msdos.h (dostounix_filename): Adjust prototype.
 src/w32.h (dostounix_filename): Adjust prototype.
 src/msdos.c (dostounix_filename): Accept an additional argument and
 ignore it.
 (init_environment): Adjust callers of dostounix_filename.
 src/fileio.c (Ffile_name_directory, file_name_as_directory)
 (directory_file_name, Fexpand_file_name)
 (Fsubstitute_in_file_name): [DOS_NT] Adjust call to
 dostounix_filename.
 [WINDOWSNT]: Downcase file names if w32-downcase-file-names is
 non-nil.
 (Fsubstitute_in_file_name): [DOS_NT] Don't downcase environment
 variables, as egetenv is case-insensitive for DOS_NT.
 src/dired.c (file_name_completion): Don't call Fdirectory_file_name
 with an encoded file name.
 src/w32proc.c (Fw32_short_file_name, Fw32_long_file_name): Adjust
 calls to dostounix_filename.
 src/w32fns.c (Fx_file_dialog): Adjust call to dostounix_filename.
 src/unexw32.c (unexec): Adjust call to dostounix_filename.
 src/termcap.c (tgetent) [MSDOS]: Adjust call to dostounix_filename.
 src/emacs.c (decode_env_path) [DOS_NT]: Adjust call to
 dostounix_filename.
 src/callproc.c (Fcall_process) [MSDOS]: Adjust call to
 dostounix_filename.