[tz] Extended data format

[Paul Eggert]

Ian Abbott wrote:
> I guess such =zone references should refer to a zone earlier in the same file to keep things simple

But order is not supposed to matter in zic input.
The man page says "Except for continuation lines,
lines may appear in any order in the input."
It's probably better to continue in that tradition,
even if it's a bit harder to implement.

I think it'd be OK if the new "extended" file is not treated
as if it could be compiled by itself.  I've often chafed
at the current approach, where we for example need to copy rules
from other files into "antarctica"; I'd rather avoid
copies in the data, as they're harder to maintain.