Handle ECN handshake in simultaneous open

While testing simultaneous open TCP with ECN, found that
negotiation fails to arrive at the expected final state.

Reviewed by: tuexen (mentor)
Approved by: tuexen (mentor), rgrimes (mentor)
MFC after: 2 weeks
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D23373