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spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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

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.

Changes to allow the patching of packets with an offset (and other changes.. see man page)

PR: 206185
Submitted by: Dmitry Vagin <daemon-hammer@ya.ru>
MFC after: 1 week
Relnotes: yes (also ng_checksum node)

slite style changes. There is an incoming patch that rewrites a
lot of this module and I want to get the style and whitespace changes in
a separate commit (or maybe more).

PR: 206185
Submitted by: Dmitry Vagin
MFC after: 1 month

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

Style(9) fixes:
* Sort includes
* Replace #define<SPACE> to #define<TAB>
* Split declarations and initializations
* Split long lines

Requested by: kib
Approved by: kib (mentor)
MFC after: 1 month

New netgraph node ng_patch(4). It performs data modification of packets
passing through. Modifications are restricted to a subset of C language
operations on unsigned integers of 8, 16, 32 or 64 bit size.
These are: set to new value (=), addition (+=), subtraction (-=),
multiplication (*=), division (/=), negation (= -), bitwise AND (&=),
bitwise OR (|=), bitwise eXclusive OR (^=), shift left (<<=),
shift right (>>=). Several operations are all applied to a packet
sequentially in order they were specified by user.

Submitted by: Maxim Ignatenko <gelraen.ua at gmail.com>
Vadim Goncharov <vadimnuclight at tpu.ru>
Discussed with: net@
Approved by: mav (mentor)
MFC after: 1 month