Change explicit uses of the Unix directory separator

Change explicit uses of the Unix directory separator
/' to uses of the macros IS_ANY_SEP, IS_DIRECTORY_SEP,
S_DEVICE_SEP, DIRECTORY_SEP, and DEVICE_SEP.

[WINDOWSNT]: Don't define HAVE_FSYNC; add includes for NT.

(Ffile_name_absolute_p): Test DOS_NT instead of MSDOS.
(Fwrite_region, Fdo_auto_save, Ffile_modes): Likewise.

(Qfind_buffer_file_type): Test DOS_NT instead of MSDOS.
(syms_of_files): Likewise.
(Finsert_file_types): Test DOS_NT instead of MSDOS.
Rename local var try to trytry.

(Fadd_name_to_file): Wlways fail.
(Frename_file) [WINDOWSNT]: Use MoveFile, not link and unlink,
and check for both ERROR_FILE_EXISTS and ERROR_ALREADY_EXISTS.
(Fmake_directory_internal) [WINDOWSNT]: Invoke mkdir without the mask.

(Fexpand_file_name): Test DOS_NT, not MSDOS.
(Fexpand_file_name) [WINDOWSNT]: Accept // or \\ at start.
Call dostonunix_filename for HOME envvar, for ~USER.
Quote directory separators found in environment variables.
(Fsubstitute_in_file_name): Test DOS_NT instead of MSDOS.
(Fsubstitute_in_file_name) [WINDOWSNT]: Accept // or \\ at start.
Work around alloca bug in MS compiler.
(Ffile_name_directory): Test DOS_NT instead of MSDOS sometimes.
But don't insert a drive letter on windows.