CL 192518 was a minimal simplification to get sendfile
on Windows to work with chunked files, but as I had mentioned,
I would add even more improvements.

This CL improves it by:
* If the reader is not an *io.LimitedReader, since the underlying
reader is anyways an *os.File, we fallback and stat that
file to determine the file size and then also invoke the chunked
sendFile on the underlying reader. This issue existed even
before the prior CL.
* Extracting the chunked TransmitFile logic and moving it directly
into internal/poll.SendFile.

Thus if the callers of net.sendFile don't use *io.LimitedReader,
but have a huge file (>2GiB), we can still invoke the chunked
internal/poll.SendFile on it directly.

The test case is not included in this patch as it requires
creating a 3GiB file, but that if anyone wants to view it, they
can find it at
    https://go-review.googlesource.com/c/go/+/194218/13/src/net/sendfile_windows_test.go

Updates #33193.

Change-Id: I97a67c712d558c84ced716d8df98b040cd7ed7f7
Reviewed-on: https://go-review.googlesource.com/c/go/+/194218
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>