[tz] Deprecating tzsetwall

In reviewing the tzdb web pages for bad links and suchlike, I noticed that they're still talking about UNIX SVR2 (superseded since 1988) and tzsetwall.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.