This fixes a load-time failure on versions of Windows too old to have
that function in kernel32.dll.
We use it to determine whether a file was safe to overwrite in the
context of PuTTY session logging: if it's safe, we skip the 'do you
want to overwrite or append?' dialog box.
On earlier Windows you can use FindFirstFile to get a similar effect,
so that's what we fall back to. It's not quite the same, though - if
you pass a wildcard then it will succeed when you'd rather it had
failed. But it's good enough to at least work in normal cases.
83ff08f9db