trpt: fix several bugs and add deprecation notice

This patch fixes several bugs releated to trpt including

• the computation of the TCP payload bytes, in particular when TCP options are present or IPv4 is used.
• the logging of incoming and outgoing packets.
• the logging of timers, in particular add support for logging of the delayed ack timer.

Also add a deprecation note, since support for this utility has been
removed from the main branch. Therefore, this is a direct commit to
stable/13.

Reviewed by: cc, glebius, rrs
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D39436