[tz] Getting current tzdb version in use

[Paul Eggert]

Robert Elz wrote:
> I have more than doubts, I am convinced that the entire thing is
> misguided, and that (with the one exception of curiosity value,
> the ability of an application to tell people which version data it
> is using) there is no good use of  this data

Yes, that's the only good use I can think of as well. That is, the version 
string is more for diagnosing obsolete or misconfigured systems than for use in 
ordinary computation. That being said, configuration is of growing importance in 
software engineering, and I hope that the version comment in tzdata.zi is enough 
to satisfy needs in this area.

With TZDIST the version info is best done via ETags, and this should work with 
TZif files so that clients can easily see whether they're up-to-date and get a 
new version if not. See Internet RFC 7808 section 4.1.4 along with:

https://tools.ietf.org/html/draft-murchison-tzdist-tzif-09