Maintain some "TSO potential" state during segmentation at
sendmsg()/sendpage() time.  Use this at push-pending-frames
time to defer tcp_write_xmit() calls and control it's behavior.

Add tcp_flush_queue() which doesn't try to optimize TSO,
it is invoked when getting packets out is more important
than producing larger TSO chunks.  These two cases are:

1) At end of sendmsg()/sendpage() call without MSG_MORE,
   indicating that we have no way to know for sure if
   the user will queue up more TCP data to send.

2) When sleeping within sendmsg()/sendpage() waiting for
   memory.  Pushing out packets and receiving the ACKs
   may very well be the event that will free up send
   queue space for us.

(Must consider interactions with Nagle and Minshall rules)

Consider tcp_opt state which keeps a "TSO goal", it
must be in sync with tcp_opt MSS state.  Initially
define "TSO goal" using tcp_tso_win_divisor and the
current congestion window.  Formally this is:

	max(1U, CWND / TCP_TSO_WIN_DIVISOR)

We could either maintain this lazily, costing us a divide
each time it is recalculated.  Or, we can update it
incrementally each time snd_cwnd is updated.

To save some state testing during output decisions,
define "TSO goal" as one for non-TSO flows.

Possible send test logic:

	if (no new data possibly coming from user)
		send_now();
	if (sending due to ACK queue advancement)
		send_now();
	send_tso_goal_sized_chunks();