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

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

spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD

The SPDX folks have obsoleted the BSD-2-Clause-FreeBSD identifier. Catch
up to that fact and revert to their recommended match of BSD-2-Clause.

Discussed with: pfg
MFC After: 3 days
Sponsored by: Netflix

netgraph/ng_nat: Add RFC 6598/Carrier Grade NAT support

This extends upon the RFC 6598 support to libalias/ipfw in r357092.

Reviewed By: manpages (bcr), donner, adrian, kp
Approved by: kp (mentor)
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D23461

ng_nat: unbreak ABI

The revision r342168 broke ABI of ng_nat needlessly and
the change was merged to stable branches breaking ABI there, too.
Unbreak it.

PR: 250722
MFC after: 1 week

Allow ng_nat to be attached to a ethernet interface directly via ng_ether(4)
or the likes. Add new control message types: setdlt and getdlt to switch
from default DLT_RAW (no encapsulation) to DLT_EN10MB (ethernet).

Approved by: glebius
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D18535

sys: general adoption of SPDX licensing ID tags.

Mainly focus on files that use BSD 2-Clause license, however the tool I
was using misidentified many licenses so this was mostly a manual - error
prone - task.

The Software Package Data Exchange (SPDX) group provides a specification
to make it easier for automated tools to detect and summarize well known
opensource licenses. We are gradually adopting the specification, noting
that the tags are considered only advisory and do not, in any way,
superceed or replace the license texts.

No functional change intended.

Add NGM_NAT_LIBALIAS_INFO command, that reports internal stats
of libalias instance. To be used in the mpd5 daemon.

Submitted by: Dmitry Luhtionov <dmitryluhtionov gmail.com>

A new jail(8) with a configuration file, to replace the work currently done
by /etc/rc.d/jail.

- Import the HEAD csup code which is the basis for the cvsmode work.

Fix incorrect field name.

Add support for the libalias redirect functionality.

Submitted by: Vadim Goncharov <vadim_nuclight@mail.ru>

Add support for setmode and settarget messages.

Approved by: glebius (mentor)

ng_nat - a netgraph(4) node, which does NAT