[tz] Correcting daylight-savings time for central Missouri prior to 1967

[Paul Eggert]

Jane Eisenstein wrote:
> I’d be satisfied if software that relies on the tz database simply warned users that DST info prior to 1970 may not be correct.

Any such warning shouldn't be limited to pre-1970 info. Another part of 
theory.html says "The tz database is not authoritative, and it surely has 
errors," and this applies to info after 1970 too. Some info we haven't had time 
to fix, some is disputed, some fixes are waiting for the next release, and some 
is incorrect info that we simply don't know is incorrect.