[tz] Rules for TZ+ database

[Paul Eggert]

Brian Inglis wrote:
> I would like to suggest that if historical zone changes can not be accomodated
> in the main zone files, perhaps they should be added as zone entries in the
> (optional) *backward* file, overriding links in either file.

Yes, that's the sort of thing we should be looking at.
Perhaps not 'backward' itself, as with the
current zic implementation links overrides zones;
but some other file like 'backward'.  This in in
both my proposal (since reverted) and Zefram's,
and I'm confident that we'll get it to work somehow.