[tz] Adding verified historic details

[Alan Barrett]

On Thu, 05 Sep 2013, Lester Caine wrote:
> We need to split the date and code to create two repositories, 
> fork the data repository and then we can get on with TZ+ ...

No, please let's not fork the database.  Lets find a way for 
the tz project to maintain all the information, in a way that 
satisfies as many people as possible, including (a) those who want 
thousands of new zones to accommodate pre-1970 time differences, 
and (b) those who want to install or use subsets of the database 
that do not have those thousands of new zones.

It seems to me that the following changes or additions would be 
sufficient:

1. Code to extract a subset of zone names according to specified 
criteria; for example, by selecting only one of several zones that 
differ pre-1970 but are identical post-1970.  Zefram has written a 
prototype of this code.

2. A process to allow OS vendors, system vendors, packagers, 
sysadmins, and even end users, to install or use only a subset of 
the data; for example, by ignoring zones that differ from other 
zones only in pre-1970 data.  Zefram has produced a prototype of 
such code.

3. (Optional) New notation in the zic input files to make it 
easier to express the fact that, after some date (or between some 
pair of dates), one zone follows exactly the same rules as another 
zone.

4. (Optional) Changes in zic and third party parsers to handle the 
input notation changes.

5. Guidelines to allow the creation of new zones that differ only 
for pre-1970 times.

6. Data for new zones that differ only for pre-1970 times.  This 
could be contributed by volunteers.

--apb (Alan Barrett)