Licensing header additions

Licensing header additions

+50 lines for every file.

The abbreviated CC0 license header was taken from the CC0 FAQ
(https://wiki.creativecommons.org/wiki/CC0_FAQ).

There's no commonly accepted way to shorten the 2-clause BSD license,
so it has to be copied in full every time.
The copyright notices are important for 2-clause BSD license,
so adding a reference to LICENCE.md was impractical.

While already there, affix a SPDX license identifier to all files.
This helps automated license identification programs along,
such as GitHub and scancode-toolkit.

License headers are useful to clarify what license conditions apply
to a particular file.