[tz] Fractional seconds in zic input

[Paul Eggert]

Howard Hinnant wrote:

> If we are to add fractional second support, it should come with a specification of the precision, or precision-range that is supported (e.g. centiseconds, milliseconds, whatever).

zic currently supports only 1-second precision in its output. It accepts any 
amount of precision on input, limited only by size of memory, and ignores 
fractional seconds.

Come to think of it, zic should round instead; I'll add that shortly.

It's not clear to me that the zic man page should specify how much precision is 
intended to be significant.

> Also, do you anticipate a similar refinement of the “SAVE” quantity?

Yes, I think the current zic man page says that both time-of-day and UT offset 
can have fractional seconds. I imagine both could be useful, for the same reason 
that to-the-second values were useful before now.

> If such changes to the database are to be made, I would much prefer they be made asap.

We should have tzdb generate both bleeding-edge format (with fractional seconds 
and negative DST offsets) and trailing-edge format (without either feature, at 
least for now). So you'll be able to test with bleeding-edge if you like. I'll 
try to publish a patch to do that soon.