[tz] Use or Apply for SPDX Licence

[Brian Inglis]

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

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