SPDX: license tags: LicenseRef-OpenZFS-ThirdParty-PublicDomain

SPDX have repeatedly rejected the creation of a tag for a public domain
dedication, as not all dedications are clear and unambiguious in their
meaning and not all jurisdictions permit relinquishing a copyright
anyway.

A reasonably common workaround appears to be to create a local
(project-specific) identifier to convey whatever meaning the project
wishes it to. To cover OpenZFS' use of third-party code with a public
domain dedication, we use this custom tag.

Further reading:
- https://github.com/spdx/old-wiki/blob/main/Pages/Legal%20Team/Decisions/Dealing%20with%20Public%20Domain%20within%20SPDX%20Files.md
- https://spdx.github.io/spdx-spec/v2.3/other-licensing-information-detected/
- https://cr.yp.to/spdx.html

Sponsored-by: https://despairlabs.com/sponsor/
Signed-off-by: Rob Norris <robn@despairlabs.com>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>

include/sys/skein.h

@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: LicenseRef-OpenZFS-ThirdParty-PublicDomain
 /*
  * Interface declarations for Skein hashing.
  * Source code author: Doug Whiting, 2008.

module/icp/algs/skein/skein.c

@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: LicenseRef-OpenZFS-ThirdParty-PublicDomain
 /*
  * Implementation of the Skein hash function.
  * Source code author: Doug Whiting, 2008.

module/icp/algs/skein/skein_block.c

@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: LicenseRef-OpenZFS-ThirdParty-PublicDomain
 /*
  * Implementation of the Skein block functions.
  * Source code author: Doug Whiting, 2008.

module/icp/algs/skein/skein_impl.h

@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: LicenseRef-OpenZFS-ThirdParty-PublicDomain
 /*
  * Internal definitions for Skein hashing.
  * Source code author: Doug Whiting, 2008.

module/icp/algs/skein/skein_iv.c

@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: LicenseRef-OpenZFS-ThirdParty-PublicDomain
 /*
  * Pre-computed Skein IVs
  *

module/icp/algs/skein/skein_port.h

@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: LicenseRef-OpenZFS-ThirdParty-PublicDomain
 /*
  * Platform-specific definitions for Skein hash function.
  *

module/zfs/vdev_draid_rand.c

@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: LicenseRef-OpenZFS-ThirdParty-PublicDomain
 /*
  * Xorshift Pseudo Random Number Generator based on work by David Blackman
  * and Sebastiano Vigna (vigna@acm.org).