File headers: SPDX License migration

Use a shorter/simpler license convention, stops the header taking so
much space.

Follow the SPDX license specification: https://spdx.org/licenses

- C/C++/objc/objc++
- Python
- Shell Scripts
- CMake, GNUmakefile

While most of the source tree has been included

- `./extern/` was left out.
- `./intern/cycles` & `./intern/atomic` are also excluded because they
  use different header conventions.

doc/license/SPDX-license-identifiers.txt has been added to list SPDX all
used identifiers.

See P2788 for the script that automated these edits.

Reviewed By: brecht, mont29, sergey

Ref D14069

doc/license/SPDX-license-identifiers.txt

@@ -0,0 +1,5 @@
+Corresponding SPDX license identifiers in the source code:
+
+Apache-2.0                Apache-2-license.txt      https://spdx.org/licenses/Apache-2.0.html
+BSD-3-Clause              BSD-3-Clause-license.txt  https://spdx.org/licenses/BSD-3-Clause.html
+GPL-2.0-or-later          GPL-license.txt           https://spdx.org/licenses/GPL-2.0-or-later.html