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spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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/dev: further 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.

Introduce wtap, the beginnings of a net80211 wlan simulator.

This introduces:

* a basic wtap interface
* a HAL, which implements an abstraction layer for implementing
different device behavious;
* A visibility plugin, which allows for control over which nodes
see other nodes (useful for mesh work.)

It doesn't yet implement sta/adhoc/hostap modes but these are quite
feasible to implement.

Monthadar uses it to do 802.11s mesh verification.

The userland tools will be committed in a follow-up commit.

Submitted by: Monthadar Al Jaberi <monthadar@gmail.com>