Use nitems() from sys/param.h

Sponsored by:	gandi.net (BSD Day Taiwan)

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.

at(1): annotate some intended switch-case fallthroughs

Reported by:	Coverity
CID:		1008191
Sponsored by:	Spectra Logic Corp

Reported by:	Coverity
CID:		1007335
MFC after:	3 days

PR:		bin/202308
Submitted by:	John Hein <z7dr6ut7gs snkmail com>
MFC after:	2 weeks

Off by default, build behaves normally.
WITH_META_MODE we get auto objdir creation, the ability to
start build from anywhere in the tree.

Still need to add real targets under targets/ to build packages.

Differential Revision:       D2796
Reviewed by: brooks imp