Thread: pgsql: Sync our copy of the timezone library with IANA releasetzcode20

Sync our copy of the timezone library with IANA release tzcode2018e.

The non-cosmetic changes involve teaching the "zic" tzdata compiler about
negative DST.  While I'm not currently intending that we start using
negative-DST data right away, it seems possible that somebody would try
to use our copy of zic with bleeding-edge IANA data.  So we'd better be
out in front of this change code-wise, even though it doesn't matter for
the data file we're shipping.

Discussion: https://postgr.es/m/30996.1525445902@sss.pgh.pa.us

Branch
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REL9_3_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/9469ebc71bab7e3beca4390d395e0ee66d462cfb

Modified Files
--------------
src/timezone/README      |   2 +-
src/timezone/localtime.c |  14 ++---
src/timezone/strftime.c  |   2 +-
src/timezone/zic.c       | 157 ++++++++++++++++++++++++++++++++++++-----------
4 files changed, 129 insertions(+), 46 deletions(-)