[tz] Fractional seconds in zic input

[Paul Eggert]

On 02/05/2018 10:46 AM, Howard Hinnant wrote:
> If two clients (different platforms) want to maintain the invariant that equal time_points remain equal after mapping, then they must operate at the precision of the mapping (or finer).
We already have clients that don't want to do that, as they discard 
sub-minute resolution. But I take your point that some clients may want 
to do that and we should cater to this subclass of clients too. In that 
case, how about if we stick to at most 1-ms resolution in the data, and 
note in zic.8 that 1 ms resolution is the way to go? I say "1 ms" 
because of Steve Allen's email.