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spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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

various: 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.

- Fix a bug that can lead to displaying an incorrect value. (r224210)
- Fix an abnormal termination caused by twice of "rtadvctl disable". (r224303)
- Use poll() to wait for the control message socket instead of a spin loop.
(r224304)
- s/cmsg_/cm_/ to avoid conflict with CMSG_* symbols for struct cmsghdr.
(r224619)
- Ignore an interface that never sent RAs for graceful shut-down. (r224620)
- Refine log messages. (r225148)
- Fix SIGSEGV when receiving RAs that contain RDNSS and/or DNSSL options.
(r225149)

Approved by: re (kib)

- Improve interface list handling. The rtadvd(8) now supports dynamically-
added/removed interfaces in a more consistent manner and reloading the
configuration file.

- Implement burst unsolicited RA sending into the internal RA timer framework
when AdvSendAdvertisements and/or configuration entries are changed as
described in RFC 4861 6.2.4. This fixes issues that make termination of the
rtadvd(8) daemon take very long time.

An interface now has three internal states, UNCONFIGURED, TRANSITIVE, or
CONFIGURED, and the burst unsolicited sending happens in TRANSITIVE.
See rtadvd.h for the details.

- rtadvd(8) now accepts non-existent interfaces as well in the command line.

- Add control socket support and rtadvctl(8) utility to show the RA information
in rtadvd(8). Dumping by SIGUSR1 has been removed in favor of it.