[tz] Add option to act like global-tz

[Paul Eggert]

On 8/17/22 10:31, Bradley White wrote:
> For what it's worth, that patch appears to remove the last instance of "DST
> all year"
> <https://github.com/eggert/tz/blob/aca1a705795d4d3986a3ac795e673c410ace2693/zic.c#L2760-L2761>.
> Hopefully
> you have some other hidden tests that continue to exercise those code paths.

Unfortunately all the tests I normally run are in the Makefile, and none 
of them exercise "DST all year" as far as I know.

It would be nice if someone had the time to write good test cases for 
"DST all year". For example, POSIX requires support for it by via tricky 
settings like TZ='XXX6PDT7,0/0,J365/23' and tzcode should work with such 
settings (which I think it does).

That being said, I expect that a lot of other software would misbehave 
under "DST all year". This is partly why tzdata has routinely modeled 
"DST all year" as standard time in the past. This may sound like a small 
thing, but it could turn into a real problem if the US adopts a "DST all 
year" law.