Remove $FreeBSD$: one-line .c comment pattern

Remove /^/[*/]\s*\$FreeBSD\$.*\n/

General further adoption of SPDX licensing ID tags.

Mainly focus on files that use BSD 3-Clause license.

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.

Special thanks to Wind River for providing access to "The Duke of
Highlander" tool: an older (2014) run over FreeBSD tree was useful as a
starting point.

- Use time_uptime instead of time_second in data structures for
PF_INET6 in kernel. This fixes various malfunction when the wall time
clock is changed. Bump __FreeBSD_version to 1000041.

- Use clock_gettime(CLOCK_MONOTONIC_FAST) in userland utilities.

MFC after: 1 month

- 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.