[tz] Getting close to new release: testing, testing, ...

[Paul Eggert]

Ian Abbott wrote:
> It ought to be documented how version-0 parsers and version-2 parsers are
> supposed to parse version-N data files.

I snuck that into one of yesterday's patches.
The latest tzfile(5) ends with:
"Future changes to the format may append more data."
That describes how version 0 parsers can parse
version 2 format, and is one plausible way that
any version 4 might evolve from version 3.

Version 3 is an exception, since it appends zero data
to version 2, and slightly extends the
interpretation of the version-2 format in a way
that seems to match existing practice already in most
cases (and the exceptions don't misbehave significantly).