MFC r326276:

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.

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Revert most part of 200420 as requested, as more review and polish is
needed.

Remove unneeded header includes from usr.bin/ except contributed code.

Tested with: make universe

Import ar(1) front-end. (aka 'BSD' ar)

Reviewed by: jkoshy
Approved by: jkoshy (mentor)
Tested by: erwin (ports build test on pointyhat)
Sponsored by: Google Summer of Code 2007
Reviewed by (earlier version): Jaakko Heinonen <jh[AT]saunalahti.fi>
Tested by (earlier version): Steve Kargl <sgk[AT]troutmask.apl.washington.edu>
Tested by (earlier version): Martin Voros <martin_voros[AT]yahoo.com>
Tested by (earlier version): swell.k[AT]gmail.com
Tested by (earlier version): joel
Tested by (earlier version): Alexey Shuvaev <shuvaev[AT]physik.uni-wuerzburg.de>
Tested by (earlier version): Arjan van Leeuwen <avleeuwen[AT]gmail.com>

Thanks to gabor@ for building ports for it.
Thanks to erwin@ and kris@ for scheduling the ports build test on pointyhat.
And thanks to many others for their feedback.