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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.