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spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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

wtap: Implement IBSS mode on wtap(4)

For TSF and beacon generation, each STA has to start its local TSF timer
and sends beacon frames when it reaches the state IEEE80211_S_RUN.
The TSF timer will be used for IBSS merge and beacon frame transmission.

The TSF timer is kept in the HAL of wtap(4). It is working by
continuously updating its value on timer interrupt simulated
by callout_reset().

When receiving beacons, the STA will be merged into the IBSS if
- the STA has the same SSID as the beacon sender
- the STA's TSF timer is smaller than the beacon sender's TSF timer.

After the merging process, the younger STA will be in the IBSS
created by the older STA and the younger STA stops sending beacon
frames. So beacon frames will always be sent by the oldest STA
in IBSS.

Sponsored by: Google, Inc. (GSoC 2022)
Reviewed By: lwhsu, adrian
Differential Revision: https://reviews.freebsd.org/D35841

wtap: clean up empty lines in .c and .h files

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>