[tz] POSIX-TZ-environment-variable-style string rules

[Paul Eggert]

Alexander Belopolsky wrote:

> I think the best way forward would be to document the fact that a tzfile
> with the last explicit transition not matching a POSIX rule transition is
> invalid and the behavior of localtime with such file is undefined.

Thanks for thinking it through. I implemented that in the attached patches, 
which also change zic.c to obey the new restriction on tzfile format. I 
installed these patches into the experimental repository on GitHub.

> I suggest fixing the issue in two steps:
>
> 1. Add 2038-03-28 transition to the Morocco rules in the raw africa file.

I don't see how that would work for 32-bit data, since 2038-03-28 is outside the 
32-bit window. Another possibility would be to remove Morocco's 2037 transitions 
to cater to now-buggy zic implementations like zic 2016f, but I'm not sure it's 
worth the hassle.

> 2. Modify zic to warn about raw files where the last explicit transition
> does not match the rule from the POSIX string and add an appropriate
> transition in the binary file.

The attached patch merely adds the appropriate transition without warning. If 
someone wants to change zic.c to emit such a warning, that'd be OK too.
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