[tz] Merged 1970+ time zones should always return -1 pre-1970

[Paul Eggert]

On 9/27/21 11:32 PM, Brian Inglis via tz wrote:
> I think for all zones with data merged prior to 1970, the data and API 
> should always return *-1* for all times prior to 1970-01-01 00:00:00Z, 
> as anything else is inaccurate or a kludge.

Although we could do that as an option, using that option would cause 
more data churn than other proposals presented.

Many Zones in tzdb are "zones with data merged prior to 1970". 
America/New_York, for example, merges many regions that did not observe 
the same time before 1970. The same is true of America/Chicago, 
Europe/Paris, Asia/Shanghai, etc. For most tzdb uses, I would say this 
merging is far more the rule than the exception.