zdump -v crashes with 64-bit time_t

[Olson, Arthur David (NIH/NCI)]

> * Don't attempt to print out all the 64-bit time_t space, as that will
>   take nearly forever; instead, default to printing about 50 years
>   into the future, and don't bother to print times before about 1833.

It might be better to cut off at something such as...
	2050 + 100 * ((current_year - 1930) / 100)
...to ease regression testing; this would maintain a cutoff of 2050 until
2030, then boost the cutoff to 2150 until 2130, and so on.
This way zdump output only changes every hundred years; the next change is
set for eight years before 2038 and the miseries associated with T4G (you
read the abbreviation here first) to provide ample time to catch bugs, while
avoiding any output changes for the next 26 years.

				--ado