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
Differential Revision: https://reviews.freebsd.org/D30740
Sponsored by: Netflix, Inc.

(cherry picked from commit 870af3f4dc57a6bbfc03f6a49ca0d5b7ff1b975a)