[tz] time keeps on slipping into the future

[Bradley White]

tzfile.5 says:
> After the second header and data comes a newline-enclosed,
> POSIX-TZ-environment-variable-style string for use in handling
> instants after the last transition time stored in the file

I had imagined that the reference implementation would apply the future
rule for all of the future, but I currently observe that UTC-offset changes
get stuck after a while for many (but not all) zones.

For example, for *"Australia/Sydney"*, whose POSIX TZ string is
*"AEST-10AEDT,M10.1.0,M4.1.0/3"*, tz_localtime_rz() produces the following
civil times at the given instants:

  14745254399 = 2437-04-05T02:59:59+1100
  14745254400 = 2437-04-05T02:00:00+1000
  14760979199 = 2437-10-04T01:59:59+1000
  14760979200 = 2437-10-04T03:00:00+1100

That looks good ... daylight time from the first Sunday in Oct through the
first Sunday in Apr, shifting at 03:00.  But once we hit 2438, the UTC
offset is stuck at +1100.  For example, in July we would expect to see
standard time, but instead get:

  14784296400 = 2438-07-01T00:00:00+1100

And it never seems to recover after that.

I'm pretty sure this used to work.
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