Turn `directory-sep-char' into a noop.

Turn `directory-sep-char' into a noop.

* lisp/subr.el (directory-sep-char): Move from fileio.c and make a defconst.

* src/emacs.c (decode_env_path): Don't check DIRECTORY_SEP,
  call dostounix_filename directly.

* src/fileio.c (CORRECT_DIR_SEPS): Remove.
  (Ffile_name_directory, directory_file_name, Fexpand_file_name)
  (Fsubstitute_in_file_name): Use dostounix_filename instead.
  (file_name_as_directory): Use dostounix_filename, DIRECTORY_SEP.
  (syms_of_fileio) <directory-sep-char>: Move to subr.el.

* src/lisp.h [WINDOWSNT] (Vdirectory_sep_char): Don't declare.
  (DIRECTORY_SEP): Define unconditionally.

* src/w32proc.c (CORRECT_DIR_SEPS): Remove.
  (Fw32_short_file_name, Fw32_long_file_name): Use dostounix_filename.

* src/s/ms-w32.h (DIRECTORY_SEP): Remove.