tcp: tolerate missing timestamps
Some TCP stacks negotiate TS support, but do not send TS at all
or not for keep-alive segments. Since this includes modern widely
deployed stacks, tolerate the violation of RFC 7323 per default.
Reviewed by: rgrimes, rrs, rscheff
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D30740
Sponsored by: Netflix, Inc.