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spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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

Add COUNTER_U64_SYSINIT() and COUNTER_U64_DEFINE_EARLY().

The aim is to reduce the boilerplate needed today to define and
initialize global counters. Also add SI_SUB_COUNTER to the sysinit
ordering.

Reviewed by: kib
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D23977

Use underscores for internal variable name to avoid conflicts.

MFC after: 1 week

sys/sys: 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.

Provide counter_ratecheck(), a MP-friendly substitution to ppsratecheck().
When rated event happens at a very quick rate, the ppsratecheck() is not
only racy, but also becomes a performance bottleneck.

Together with: rrs, jtl

- While we are spreading the counter(9) across network stack, more userland
tools would need to know about the counter_u64_t type. Allow to include
sys/counter.h from userspace.
- Utilize now defined type in kvm_counter_u64_fetch().

Sponsored by: Netflix
Sponsored by: Nginx, Inc.

Add several macros to help migrate statistics structures to PCPU counters.

Merge from projects/counters: counter(9).

Introduce counter(9) API, that implements fast and raceless counters,
provided (but not limited to) for gathering of statistical data.

See http://lists.freebsd.org/pipermail/freebsd-arch/2013-April/014204.html
for more details.

In collaboration with: kib
Reviewed by: luigi
Tested by: ae, ray
Sponsored by: Nginx, Inc.