tcp: ECN preparations for ECN++, AccECN (tcp_respond)

tcp: ECN preparations for ECN++, AccECN (tcp_respond)

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.

Also ensure that on receiving multiple ECN SYN-ACKs, the
responses triggered will reflect the latest state.

Reviewed By:		tuexen, #transport
Sponsored by:		NetApp, Inc.
Differential Revision:	https://reviews.freebsd.org/D36973