tcp: improve behaviour when using TCP_NOOPT

Use ISS for SEG.SEQ when sending a SYN-ACK segment in response to
an SYN segment received in the SYN-SENT state on a socket having
the IPPROTO_TCP level socket option TCP_NOOPT enabled.

Reviewed by: rscheff
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D28656