If on Windows, we require the 'largefile' resource.

If not on Windows, we use a test that actually writes a byte beyond
the 2BG limit -- seeking alone is not sufficient, since on some
systems (e.g. Linux with glibc 2.2) the sytem call interface supports
large seek offsets but not all filesystem implementations do.

Note that on Windows, we do not use the write test: on Win2K, that
test can take a minute trying to zero all those blocks on disk, and on
Windows our code always supports large seek offsets (but again, not
all filesystems do).  This may mean that on Win95, or on certain other
backward filesystems, test_largefile will *fail*.