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spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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 copyrights that I forgot to add when splitting arb.h off from tree.h.
While here clean up the RCS tags.

Suggested by: lstewart
MFC after: 2 weeks
Sponsored by: Klara Inc, Netflix

Introduce stats(3), a flexible statistics gathering API.

This provides a framework to define a template describing
a set of "variables of interest" and the intended way for
the framework to maintain them (for example the maximum, sum,
t-digest, or a combination thereof). Afterwards the user
code feeds in the raw data, and the framework maintains
these variables inside a user-provided, opaque stats blobs.
The framework also provides a way to selectively extract the
stats from the blobs. The stats(3) framework can be used in
both userspace and the kernel.

See the stats(3) manual page for details.

This will be used by the upcoming TCP statistics gathering code,
https://reviews.freebsd.org/D20655.

The stats(3) framework is disabled by default for now, except
in the NOTES kernel (for QA); it is expected to be enabled
in amd64 GENERIC after a cool down period.

Reviewed by: sef (earlier version)
Obtained from: Netflix
Relnotes: yes
Sponsored by: Klara Inc, Netflix
Differential Revision: https://reviews.freebsd.org/D20477

Rename ARB_REBALANCE(3) to ARB_REINSERT(3) to match tree(3),
and document it.

MFC after: 2 weeks
Sponsored by: Klara Inc, Netflix

Introduce arb(3), the Array-based Red-Black Tree macros: similar
to the traditional tree(3) RB trees, but using an array (preallocated,
linear chunk of memory) to store the tree.

This avoids allocation overhead, improves memory locality,
and makes it trivially easy to share/transfer/copy the entire tree
without the need for marshalling. The downside is that the size
is fixed at initialization time; there is no mechanism to resize
it.

This is one of the dependencies for the new stats(3) framework
(https://reviews.freebsd.org/D20477).

Reviewed by: bcr (man pages), markj
Discussed with: cem
MFC after: 2 weeks
Sponsored by: Klara Inc, Netflix
Obtained from: Netflix
Differential Revision: https://reviews.freebsd.org/D20324