[tftp] Allow TFTP block size to be controlled via the PXE TFTP API

The PXE TFTP API allows the caller to request a particular TFTP block
size.  Since mid-2008, iPXE has appended a "?blksize=xxx" parameter to
the TFTP URI constructed internally; nothing has ever parsed this
parameter.  Nobody seems to have cared that this parameter has been
ignored for almost five years.

Fix by using xfer_window(), which provides a fairly natural way to
convey the block size information from the PXE TFTP API to the TFTP
protocol layer.

Signed-off-by: Michael Brown <mcb30@ipxe.org>