Sync timezone code with tzcode 2010c from the Olson group. This fixes some

corner cases that come up in certain timezones (apparently, only those with
lots and lots of distinct TZ transition rules, as far as I can gather from
a quick scan of their archives).  Per suggestion from Jeevan Chalke.

Back-patch to 8.4.  Possibly we need to push this into earlier releases
as well, but I'm hesitant to update them to the 64-bit tzcode without
more thought and testing.