* Switch to yearless copyright per FB policy
* Fix up SPDX-License-Identifier lines in `contrib/linux-kernel` sources
* Add zstd copyright/license header to the `contrib/linux-kernel` sources
* Update the `tests/test-license.py` to check for yearless copyright
* Improvements to `tests/test-license.py`
* Check `contrib/linux-kernel` in `tests/test-license.py`
Add the kernel wrapper API. This keeps the same API and semantics as the
existing kernel API with name changes to be more kernel style and avoid
symbol collisions with zstd.
Add decompress_sources.h, which includes all the decompression .c files.
This is used for kernel decompression.
Also, add a test which checks that including decompress_sources.h works.
