[tz] [PROPOSED] Support zi parsers that mishandle negative DST offsets

[Paul Eggert]

On February 9 I wrote:

> Assuming the current development version, the process could run something like 
> the following, say.

The usual springtime DST uproar is behind us, so now is a good time to work out 
the kinks in this process. I installed into the GitHub development repository 
the attached proposed patches to automate an improved version of the process. 
Turning the crank on this automation results in the gzipped tarball cited below. 
This tarball does not use negative DST offsets, so it can be used by parsers 
that do not support negative DST offsets; otherwise this tarball is like the 
regular tarballs (the idea is that these will resume using negative DST offsets 
a la tzdb 2018b, this time for Africa/Windhoek, Europe/Dublin, and Europe/Prague).

In short, please try out your tzdata parsers on this test tarball:

https://web.cs.ucla.edu/~eggert/tz/test/tzdata2018d-22-g89da66c-rearguard.tar.gz

It has an associated .asc signature as usual. Also, its parent directory has the 
corresponding files that we already generate, if you'd like to compare rearguard 
to main data.
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