Add new public domain SPDX identifiers from DJB

[pombredanne]

https://cr.yp.to/spdx.html introduces new public domain SPDX license refs:

SPDX identifiers for simple public-domain dedications

The Software Package Data Exchange (SPDX) says that it is an "open standard for communicating software bill of material information, including provenance, license, security, and other related information".

The SPDX license list includes standard names for some complicated public-domain dedications, such as CC0-1.0. Unfortunately, at the time of this writing (January 2025), the license list does not include standard names for simple public-domain dedications (such as the simple text "Public domain"), even when those dedications are commonly used. (The closest is SAX-PD-2.0, which is a relatively complicated dedication.)

Fortunately, SPDX is user-extensible. This page defines the following names for the following simple public-domain dedications:

• LicenseRef-PD-2: "Public domain."
• LicenseRef-PD-d: "This work is dedicated to the public domain."
• LicenseRef-PD-hd: "This work is hereby dedicated to the public domain."
• LicenseRef-PD-hp: "This work is hereby placed into the public domain."

All of these dedications are already commonly used, modulo substitutions of specific names for "this work". Perhaps these will be added to the standard SPDX license list as PD-2, PD-d, PD-hd, PD-hp respectively, following SPDX's decision to "treat each unique 'Public Domain dedication' in the same way as any license is treated in the standard". In any case, for continuity, this page will continue to define LicenseRef-PD-2 etc.

For comparison, LicenseRef-Fedora-Public-Domain is a generic name for many different texts, https://gitlab.com/fedora/legal/fedora-license-data/-/blob/main/public-domain-text.txt contrary to Annex B of the SPDX specification.

Related reading: See "Placing work into the public domain" for information (including specific quotes from court decisions) on the legal effect of simple public-domain dedications.

We should also validate that we detect correctly all licenses in https://gitlab.com/fedora/legal/fedora-license-data/-/blob/main/public-domain-text.txt

[pombredanne]

It is slightly problematic, as this creates subtle language distinctions that unlikely to have any different impact from a legal PoV IMHO. We could create dedicated licenses, but this would make this murky and create havoc on tons of rules.