pgsql: Sync our copy of the timezone library with IANA release tzcode20

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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master

Details
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https://git.postgresql.org/pg/commitdiff/b45f6613e0a475f908d93dbaa8612ccb9395f666

Modified Files
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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(-)