On Windows, `mmap.mmap(f.fileno(), ...)` has the undocumented side effect of setting f's file pointer to 0.

The responsible code in mmapmodule is this:

    /* Win9x appears to need us seeked to zero */
    lseek(fileno, 0, SEEK_SET);

Win9x is no longer supported, and I'm quite sure that NT doesn't have whatever problem they were trying to fix. I think this code should be deleted, and a regression test added to verify that mmap leaves the file pointer alone on all platforms.

(mmap also maintains its own file pointer, the `pos` field of `mmap_object`, which is initially set to zero. This issue is about the kernel file pointer, not mmap's pointer.)