more ECN preparations for ECN++, AccECN (tcp_respond)

[rscheff]

Summary

tcp_respond is another function to build a tcp control packet
quickly. With ECN++ and AccECN, both the IP ECN header, and
the TCP ECN flags are supposed to reflect the correct state.

While at it, also ensure that on receiving multiple
ECN <SYN,ACK>s, the responses triggered reflect the latest
inbound state (IP, TCP ECN) - no longer assume the tcpcb
ECN flags start off "clean", but set them metriciously.
adjust function definition

Test Plan

Note how the AccECN flags of each final 3WHS ACK adjust to the IP ECN and
inbound <SYN,ACK> ECN flags change.

`
sysctl -q net.inet.tcp.ecn.enable=3
`

    0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 4
+.002 ... 0.052 connect(4, ..., ...) = 0

+.002 > [noecn] SEWA 0:0(0) <mss 1460,nop,wscale 6,sackOK,TS val 100 ecr 0>
+0.05 < [noecn] SW. 0:0(0) ack 1 win 32767 <mss 1460,sackOK,nop,wscale 8,eol,eol>
+.002 > [noecn] W. 1:1(0) ack 1

+0.05 < [noecn] SW. 0:0(0) ack 1 win 32767 <mss 1460,sackOK,nop,wscale 8,eol,eol>
+.002 > [noecn]  W. 1:1(0) ack 1

+0.05 < [ect0]  SW. 0:0(0) ack 1 win 32767 <mss 1460,sackOK,nop,wscale 8,eol,eol>
+.002 > [noecn]  A. 1:1(0) ack 1

+0.05 < [ect1]  SW. 0:0(0) ack 1 win 32767 <mss 1460,sackOK,nop,wscale 8,eol,eol>
+.002 > [noecn] EW. 1:1(0) ack 1

+0.05 < [ce]    SW. 0:0(0) ack 1 win 32767 <mss 1460,sackOK,nop,wscale 8,eol,eol>
+.002 > [noecn] WA. 1:1(0) ack 1

+0.05 < [noecn] SE. 0:0(0) ack 1 win 32767 <mss 1460,sackOK,nop,wscale 8,eol,eol>
+.002 > [noecn]   . 1:1(0) ack 1

+0.05 < [noecn]  S. 0:0(0) ack 1 win 32767 <mss 1460,sackOK,nop,wscale 8,eol,eol>
+.002 > [noecn]   . 1:1(0) ack 1

[rscheff]

• only access exsting tp

[rscheff]

didn't automatically close after commit.