[tz] Fractional seconds in zic input

[Paul Eggert]

For many years I've chafed at tzdata's lack of support for fractional seconds. 
We have good evidence of well-established standard-time UT offsets that were not 
multiples of one second; for example, the Netherlands before 1937. These can't 
be recorded in tzdata except as comments.

Ideally tzcode would support fractional-second times and UT offsets all the way 
down the chain. This would mean changes to the tz binary format and to the 
runtime API, though, which is not something we'd do lightly, if ever. However, 
it's easy to change the zic spec to allow fractional seconds, and to change zic 
to accept and ignore the fractions, so that fractional seconds can be documented 
more formally in the data; this could well be useful to applications other than 
tzcode. Proposed patch attached, hairy sscanf format and all.

This patch does not actually change the data, as we'll need time, and/or a 
procedure to automatically generate data compatible with zic 2018c and earlier.
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