[tz] Deprecating tzsetwall

[Paul Eggert]

In reviewing the tzdb web pages for bad links and suchlike, I noticed that 
they're still talking about UNIX SVR2 (superseded since 1988) and tzsetwall. So 
I looked into tzcode's implementation of tzsetwall, and noticed that it does 
nothing useful because functions like localtime now call tzset (as POSIX has 
require for many years), thus undoing tzsetwall's effect.

Since nobody evidently cares whether tzsetwall works, we should remove it. Not 
right away of course, but we can document it as deprecated now without changing 
the implementation, and remove it in a future tzcode release. So I installed the 
attached patch to do that. Comments welcome as usual.
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