[tz] zdump new option -i for easier-to-review output

[Paul Eggert]

Jon Skeet wrote:
> Given the other reactions around file merging, perhaps the data file should
> just be hosted as a separate file?

Hmm, I think I'd rather not ship *three* tarballs per release for the indefinite 
future. *Two* tarballs are already too many.

As the new .tzs file is closely associated with the data, it belongs in the 
tzdata tarball when we're talking about old-format distributions.

By the way, I picked a 2050 cutoff date for the draft .tzs partly because I 
wanted to check a few years past the 2038 limit imposed by 32-bit signed time_t. 
I didn't go too much past 2050 partly to cut down on bloat, and partly because 
the reference implementation of zdump is too slow.