[tz] tzfiles contain Unix epoch for the first transition time

[Paul Eggert]

Eric Erhardt wrote:
> 2. Will there ever be more than one transition time that is before January 1, 0001?  Or will the "big bang" transition be the only one?

It's unlikely that we'll see more than one transition before that cutoff in the 
published data.  That being said, the binary file format does allow more than 
one such transition and it should be easy enough to ignore all but the last one. 
  We were toying with the idea having zic put one transition at -2**63 and a 
later transition at the Big Bang, for example, and I'd rather not rule that out 
in future versions.