khelp: uma_zcreate() does not fail

No functional change intended.

MFC after: 1 week

sys: Automated cleanup of cdefs and other formatting

Apply the following automated changes to try to eliminate
no-longer-needed sys/cdefs.h includes as well as now-empty
blank lines in a row.

Remove /^#if.*\n#endif.*\n#include\s+<sys/cdefs.h>.*\n/
Remove /\n+#include\s+<sys/cdefs.h>.*\n+#if.*\n#endif.*\n+/
Remove /\n+#if.*\n#endif.*\n+/
Remove /^#if.*\n#endif.*\n/
Remove /\n+#include\s+<sys/cdefs.h>\n#include\s+<sys/types.h>/
Remove /\n+#include\s+<sys/cdefs.h>\n#include\s+<sys/param.h>/
Remove /\n+#include\s+<sys/cdefs.h>\n#include\s+<sys/capsicum.h>/

Sponsored by: Netflix

khelp: suppress useless warning message on shutdown

If a module (e.g. the ertt hhook for TCP) can't clean up at
shutdown, there is nothing to be done about it. In the ertt case,
cleanup is just shutting down a UMA zone, which doesn't need to be
done. Suppress EBUSY warnings on shutdown.

PR: 271677
Reviewed by: tuexen, imp
Differential Revision: https://reviews.freebsd.org/D42650

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

Remove /^[\s*]*__FBSDID\("\$FreeBSD\$"\);?\s*\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spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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

sys/kern: 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.

Cleanup and simplification in khelp_{register|deregister}_helper(). No
functional changes.

MFC after: 1 week

Add a private KPI between hhook and khelp that allows khelp modules to insert
hook functions into hhook points which register after the modules were loaded -
potentially useful during boot or if hhook points are dynamically registered.

MFC after: 1 week

Internalise handling of virtualised hook points inside
hhook_{add|remove}_hook_lookup() so that khelp (and other potential API
consumers) do not have to care when they attempt to (un)hook a particular hook
point identified by id and type.

Reviewed by: scottl
MFC after: 1 week

Use the full and proper company name for Swinburne University of Technology
throughout the source tree.

Requested by: Grenville Armitage, Director of CAIA at Swinburne University of
Technology
MFC after: 3 days

- Introduce the Hhook (Helper Hook) KPI. The KPI is closely modelled on pfil(9),
and in many respects can be thought of as a more generic superset of pfil.
Hhook provides a way for kernel subsystems to export hook points that Khelp
modules can hook to provide enhanced or new functionality to the kernel. The
KPI has been designed to ensure hook points pose no noticeable overhead when
no hook functions are registered.

- Introduce the Khelp (Kernel Helpers) KPI. Khelp provides a framework for
managing Khelp modules, which indirectly use the Hhook KPI to register their
hook functions with hook points of interest within the kernel. Khelp modules
aim to provide a structured way to dynamically extend the kernel at runtime in
an ABI preserving manner. Depending on the subsystem providing hook points, a
Khelp module may be able to associate per-object data for maintaining relevant
state between hook calls.

- pjd's Object Specific Data (OSD) KPI is used to manage the per-object data
allocated to Khelp modules. Create a new "OSD_KHELP" OSD type for use by the
Khelp framework.

- Bump __FreeBSD_version to 900028 to mark the introduction of the new KPIs.

In collaboration with: David Hayes <dahayes at swin edu au> and
Grenville Armitage <garmitage at swin edu au>
Sponsored by: FreeBSD Foundation
Reviewed by: bz, others along the way
MFC after: 3 months