[tz] Adding verified historic details

[Paul Eggert]

I agree that changing the format of
the zic input would make it considerably
easier to maintain data, particularly
the mass of pre-1970 data you're thinking
about.  But someone would need to come up with
what the new input format actually is, and implement
that change.

I expect that we would use the
new zic-input format only in the new
"extended" zones that would be excluded
from typical POSIX installations;
this should lessen backward compatibility concerns.