[tz] data not represented in tzfiles

[Zefram]

Paul Eggert wrote:
>                                       Since we're already
>relying on other extensions to POSIX as part of the recent changes,
>it should be OK to rely on this one as well,

If you're going to extend POSIX TZ as much as necessary, it can be
extended much more cleanly than this.  Allowing a wider range of
time-of-day in the TZ format is qualitatively less troublesome than
defining entirely new semantics.

-zefram