[tz] Use or Apply for SPDX Licence

Hi folks, It would be useful to distributions, packagers, and organization licence checkers if the tzcode and tzdata were covered by (a) Public Domain SPDX licence identifier(s) https://spdx.org/licenses/ e.g. CC-PDDC, PDDL, SAX-PD, Unlicense, WTFPL, etc. or applied for it's own e.g. IANA-TZ-PD. -- Take care. Thanks, Brian Inglis, Calgary, Alberta, Canada This email may be disturbing to some readers as it contains too much technical detail. Reader discretion is advised. [Data in IEC units and prefixes, physical quantities in SI.]