As reported on the transport@ and current@ mailing lists, the FreeBSD TCP
stack is not compliant with RFC 7323, which requires that TCP stacks send
a timestamp option on all packets (except, optionally, RSTs) after the
session is established.

This patch adds that support. It also adds a TCP signature option to the
packet, if appropriate.

PR: 206047
Differential Revision: https://reviews.freebsd.org/D4808
Reviewed by: hiren
MFC after: 2 weeks
Sponsored by: Juniper Networks