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.

programs/windres/verrsrc.h

@@ -1,3 +1,12 @@
+/*
+ * Copyright (c) 2016-2020, Yann Collet, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under both the BSD-style license (found in the
+ * LICENSE file in the root directory of this source tree) and the GPLv2 (found
+ * in the COPYING file in the root directory of this source tree).
+ * You may select, at your option, one of the above-listed licenses.
+ */
 /* minimal set of defines required to generate zstd.res from zstd.rc */
 
 #define VS_VERSION_INFO         1