How the new TCP output machine [nyi] works.


Data is kept on a single queue. The skb->users flag tells us if the frame is
one that has been queued already. To add a frame we throw it on the end. Ack
walks down the list from the start.

We keep a set of control flags


	sk->tcp_pend_event

		TCP_PEND_ACK			Ack needed
		TCP_ACK_NOW			Needed now
		TCP_WINDOW			Window update check
		TCP_WINZERO			Zero probing


	sk->transmit_queue		The transmission frame begin
	sk->transmit_new		First new frame pointer
	sk->transmit_end		Where to add frames

	sk->tcp_last_tx_ack		Last ack seen
	sk->tcp_dup_ack			Dup ack count for fast retransmit


Frames are queued for output by tcp_write. We do our best to send the frames
off immediately if possible, but otherwise queue and compute the body
checksum in the copy. 

When a write is done we try to clear any pending events and piggy back them.
If the window is full we queue full sized frames. On the first timeout in
zero window we split this.

On a timer we walk the retransmit list to send any retransmits, update the
backoff timers etc. A change of route table stamp causes a change of header
and recompute. We add any new tcp level headers and refinish the checksum
before sending.