MFC r326276:

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.

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Stop including if_var.h from userland.

Sponsored by: Nginx, Inc.

- 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

- 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.

- Refactoring the interface list. It now supports dynamically
added/removed interfaces in a more consistent manner and reloading
the configuration file.

- Add initial support for control socket. RA information in the
daemon can be obtained by rtadvctl(8) instead of SIGUSR1 in a similar
manner to ifconfig(8). The information dump has been removed in favor of it.

(reload the configuration file)
# rtadvctl reload

(show RA messages being sent on each interfaces)
# rtadvctl show
em0: flags=<UP,CONFIGURED,PERSIST> status=<RA_SEND> mtu 1280
DefaultLifetime: 30m
MinAdvInterval/MaxAdvInterval: 3m20s/3m20s
AdvLinkMTU: <none>, Flags: O, Preference: medium
ReachableTime: 0s, RetransTimer: 0s, CurHopLimit: 64
AdvIfPrefixes: yes

(show RA messages being sent only on em0)
# rtadvctl show em0

(rtadvctl -v show provides additional information)
# rtadvctl -v show em0
em0: flags=<UP,CONFIGURED,PERSIST> status=<RA_SEND> mtu 1280
DefaultLifetime: 30m
MinAdvInterval/MaxAdvInterval: 3m20s/3m20s
AdvLinkMTU: <none>, Flags: O, Preference: medium
ReachableTime: 0s, RetransTimer: 0s, CurHopLimit: 64
AdvIfPrefixes: yes
Prefixes (1):
2001:db8:1::/64 (CONFIG, vltime=30d, pltime=7d, flags=LA)
RDNSS entries:
2001:db8:1::128 (ltime=2m40s)

(stop rtadvd)
# rtadvctl shutdown

A remaining issue when reloading the configuration file is that
during that period rtadvd cannot communicate with rtadvctl due to some
additional RA sending for graceful shutdown. This will be fixed later.