Add missing SPDX-License tags (#591)

Add missing SPDX-License tags (#591)

* Add missing SPDX-License tags

If copyright block is present, add missing SPDX-License tag

If copyright block is not present, add one. Use BSD-3-Clause and infer author list from git blame.

Resolves #570

* Ensure all non-copyrighted files are BSD-2

Overall project is BSD-2, not 3, so ensure any new license header was BSD-2.

General formatting cleanup.

* Update md5.h to put SPDX copyright in correct place

Brain fart, I put it right before the line containing "copyright" but that was in the middle of a sentence.