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tcp: remove support for TCPPCAP

This feature could be used to store the last sent and received TCP
packets for a TCP endpoint. There was no utility to get these packets
from a live system or core.
This functionality is now provided by TCP Black Box Logging, which also
stores additional events. There are tools to get these traces from a
live system or a core.
Therefore remove TCPPCAP to avoid maintaining it, when it is not
used anymore.

Reviewed by:            rrs, rscheff, Peter Lei, glebiu
Sponsored by:           Netflix, Inc.
Differential Revision:  https://reviews.freebsd.org/D49589

sys: Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

Similar commit in current:
(cherry picked from commit 95ee2897e98f)

sys: Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

The TCPPCAP debugging feature caches recently-used mbufs for use in
debugging TCP connections. This commit provides a mechanism to free those
mbufs when the system is under memory pressure.

Because this will result in lost debugging information, the behavior is
controllable by a sysctl. The default setting is to free the mbufs.

Reviewed by:    gnn
Approved by:    re (gjb)
Differential Revision:  https://reviews.freebsd.org/D6931
Input from:     novice_techie.com

Catch up with head (r291075).

Merge from head

Sponsored by:   Gandi.net

Merge from head

MFH to r289370

Sponsored by:   The FreeBSD Foundation

There are times when it would be really nice to have a record of the last few
packets and/or state transitions from each TCP socket. That would help with
narrowing down certain problems we see in the field that are hard to reproduce
without understanding the history of how we got into a certain state. This
change provides just that.

It saves copies of the last N packets in a list in the tcpcb. When the tcpcb is
destroyed, the list is freed. I thought this was likely to be more
performance-friendly than saving copies of the tcpcb. Plus, with the packets,
you should be able to reverse-engineer what happened to the tcpcb.

To enable the feature, you will need to compile a kernel with the TCPPCAP
option. Even then, the feature defaults to being deactivated. You can activate
it by setting a positive value for the number of captured packets. You can do
that on either a global basis or on a per-socket basis (via a setsockopt call).

There is no way to get the packets out of the kernel other than using kmem or
getting a coredump. I thought that would help some of the legal/privacy concerns
regarding such a feature. However, it should be possible to add a future effort

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