opener: force copy on 'a'ppend if nlinks() returns 0 (issue1922)
If pywin32 is not installed, 'os.lstat(pathname).st_nlink' is used for
nlinks(), which is always zero for all files on Windows.
To make sure we break up hardlinks if pywin32 is missing, we force
nlink = 2 if nlinks() returns < 1.
(this completely fixes issue1922)