[tz] proposal for new tzdb versions

[Paul Eggert]

In light of the previous discussions and the fact that we need a new 
release very soon, I propose the following:

* We release 2021b pretty much as-is (with the usual release 
administrivia such as updating NEWS).

* We also generate a separate 2021a1 version, which is like 2021a except 
with the Samoa change that is prompting 2021b. This version recognizes 
the concerns about the number of changes to pre-1970 timestamps in 
2021b. I'll do this by publishing a patch to 2021a, along with a patched 
tarball, on my website at UCLA.

Although this is effectively a fork in the short term, the idea is that 
it's a small fork with the intent that we'll work together to combine 
the two approaches in later releases, taking the abovementioned concerns 
into account.

There is precedent for this approach, in that when there were 
compatibility problems with earlier releases, I generated alternate 
tarballs to support downstream users while they were adapting their 
database readers. Although the two cases are not the same, generating an 
alternate distribution also has the benefit of giving us time.

Although I haven't had time to read all the discussions so far (and 
email is still rolling in), I will try to take these discussions into 
account when writing the NEWS entries for the two versions.

After the versions are published and the dust has settled, I hope that 
we can incorporate some of the suggestions that have been made, as we 
will then have time to implement and test them. I don't want this 
followup discussion to take a looong time, though, as the goal is to 
combine the two approaches soon.

Since Samoa's rules change in less than 72 hours I plan to generate 
these new versions soon.