Fix conjunction in tcp_output_really_helper, adjust to specification

[dinosaure]

Not sure if this is the right patch, but we (/cc @reynir) spent the day looking at Sequences and Acknowledge numbers to realize that at one point, the unikernel's Sequence was decrementing. The comment above specifies that this should be the case if we receive FIN, which is not our case. I think that, due to the associativity of the || and && operators, the result of the condition becomes true and µTCP decrements the Sequence when it shouldn't. So we've just added parenthesis (by feelings).

With this fix, we can do a test with iperf3 of our unikernel (which claims a speed of 2Mbytes/s... but that's another problem).

[hannesm]

Thanks a lot, I'll review this next week.

[hannesm]

From the specification:

    let snd_nxt' = if FIN /\ (cb.snd_nxt + LENGTH data_to_send = last_sndq_data_seq+1 /\
                      cb.snd_una  <> cb.iss \/ Num(cb.snd_nxt - cb.iss) = 2) then
		     cb.snd_nxt - 1
                   else
		     cb.snd_nxt in

So, there's a parenthesis -- but it is right after the FIN && -- would the suggestion below work for you as well?

[hannesm]

Thanks a lot for your investigations and the proposed fix!