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

Sync our copy of the timezone library with IANA release tzcode2018g.

This patch absorbs an upstream fix to "zic" for a recently-introduced
bug that made it output data that some 32-bit clients couldn't read.
Given the current source data, the bug only manifests in zones with
leap seconds, which we don't generate, so that there's no actual
change in our installed timezone data files from this. Still, in
case somebody uses our copy of "zic" to do something else, it seems
best to apply the fix promptly.

Also, update the README's notes about converting upstream code to
our conventions.

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/10bfda06176739faedf909c9aba9823cdd21165d

Modified Files
--------------
src/timezone/README | 12 +++++++--
src/timezone/zic.c | 74 ++++++++++++++++++++++++++++-------------------------
2 files changed, 49 insertions(+), 37 deletions(-)