Fix copyright and license lines

* All copyright lines now have -2020 instead of -present
* All copyright lines include "Facebook, Inc"
* All licenses are now standardized

The copyright in `threading.{h,c}` is not changed because it comes from
zstdmt.

The copyright and license of `divsufsort.{h,c}` is not changed.

lib/common/cpu.h

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2018-present, Facebook, Inc.
+ * Copyright (c) 2018-2020, Facebook, Inc.
  * All rights reserved.
  *
  * This source code is licensed under both the BSD-style license (found in the