On Win2K it thought 'foo' started at byte offset 0 instead of at the
pagesize, and on Win98 it thought 'foo' didn't exist at all.  Somehow
or other this is related to the new "in memory file" gimmicks in
bsddb, but the old bsddb we use on Windows sucks so bad anyway I don't
want to bother digging deeper.  Flushing the file in test_mmap after
writing to it makes the problem go away, so good enough.