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

Sync our copy of the timezone library with IANA release tzcode2019b.

A large fraction of this diff is just due to upstream's somewhat
random decision to rename a bunch of internal variables and struct
fields. However, there is an interesting new feature in zic:
it's grown a "-b slim" option that emits zone files without 32-bit
data and other backwards-compatibility hacks. We should consider
whether we wish to enable that.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f285322f9cd3145ea2e5b870e6ba7e0c641422ac

Modified Files
--------------
src/timezone/README | 7 +-
src/timezone/localtime.c | 89 +++++-----
src/timezone/pgtz.h | 6 +-
src/timezone/tzfile.h | 17 +-
src/timezone/zic.c | 453 +++++++++++++++++++++++++----------------------
5 files changed, 305 insertions(+), 267 deletions(-)