boottrace: Use NULL for SYSINIT's last arg, which is a pointer type

MFC after: 3 days

boottrace: Add corresponding sysctl knob for loader tunable

The loader tunable 'kern.boottrace.table_size' does not have
corresponding sysctl MIB entry. Add it so that it can be retrieved,
and `sysctl -T` will also report it correctly.

Reviewed by: markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D42138

spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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

boottrace: a simple boot and shutdown-time tracing facility

Boottrace is a facility for capturing trace events during boot and
shutdown. This includes kernel initialization, as well as rc. It has
been used by NetApp internally for several years, for catching and
diagnosing slow devices or subsystems. It is driven from userspace by
sysctl interface, and the output is a human-readable log of events
(kern.boottrace.log).

This commit adds the core boottrace functionality implementing these
interfaces. Adding the trace annotations themselves to kernel and
userland will happen in follow-up commits. A future commit will also add
a boottrace(4) man page.

For now, boottrace is unconditionally compiled into the kernel but
disabled by default. It can be enabled by setting the
kern.boottrace.enabled tunable to 1 in loader.conf(5).

There is an existing boot-time event tracing facility, which can be
compiled into the kernel with 'options TSLOG'. While there is some
functional overlap between this and boottrace, they are distinct. TSLOG
is suitable for generating detailed timing information and flamegraphs,
and has been used to great success by cperciva@ to diagnose and reduce
the overall system boot time. Boottrace aims to more quickly provide an
overview of timing and resource usage of the boot (and shutdown) process
to a sysadmin who requires this knowledge.

Sponsored by: NetApp, Inc.
Sponsored by: Klara, Inc.
X-NetApp-PR: #23
Differential Revision: https://reviews.freebsd.org/D30184