[21.4] Enable win32 filenames under cygwin

DDE support under cygwin is only partially functional because the cygwin build doesn't understand win32 paths. This patch fixes it by selectively enabling the win32 filename support whilst preserving the cygwin (UNIX) semantic support. There is no new code here so I would like for this to go into 21.4 since it fixes an actual bug and I want the netinstaller to do DDE setup. andy 2001-09-08 Andy Piper <andy(a)xemacs.org&gt; * fileio.c (normalize_filename): copied from nt.c * (Ffile_name_directory): enable win32 paths under cygwin. * (Ffile_name_nondirectory): ditto. * (directory_file_name): ditto. * (Fexpand_file_name): ditto. * (Ffile_truename): ditto. * (Fsubstitute_in_file_name): ditto. * (Ffile_name_absolute_p): ditto. * (Ffile_readable_p): ditto.