[tz] [PROPOSED] Improve leapseconds support

[Hal Murray]

eggert at cs.ucla.edu said:
> * NEWS, leapseconds.awk: Add support for leap seconds at the ends of months
> other than June and December. 

A leap second other than June or December is probably an error in the input 
data.

I would leave the check and add a big comment.  Maybe write the code so that 
it is a simple edit to disable the check.

I think there is provision for an intermediate stage between Jun/Dec and every month.  Apr/Jun/Sep/Dec.

-- 
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