cygwin: unlink: workaround NFS non-ability to move file in certain cases

cygwin: unlink: workaround NFS non-ability to move file in certain cases

Under some not quite clear conditions, NFS fails to use its
unlink workaround to rename a file to ".nfsXYZ".  The problem has been
reproduced with the GAWK testext.awk testcase.  To workaround this in
Cygwin, we now call try_to_bin on NFS, too.  For some reason NFS doesn't
fail to rename the .cygXYZ file to .nfsXYZ after this Cygwin rename.
Fix comment in unlink_nt accordingly.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>