[tz] Use or Apply for SPDX Licence

[Kim Davies]

Quoting Brian Inglis on Saturday June 20, 2020:
> 
> 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) WTFPL, etc. or applied for it's own e.g. IANA-TZ-PD.

In response to a few different requests that the IANA registries have
more explicit licensing, we've been working with the IETF and IETF Trust
recently on developing an approach to licensing that preserves the current
nature but makes it clear. We'll take this tagging into consideration.

kim