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

Sync our copy of the timezone library with IANA release tzcode2016h.

This absorbs a fix for a symlink-manipulation bug in zic that was
introduced in 2016g. It probably isn't interesting for our use-case,
but I'm not quite sure, so let's update while we're at it.

Branch
------
REL9_6_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/845a8ea3e144c3a9ca2c95fd70de89c26422fb35

Modified Files
--------------
src/timezone/zic.c | 76 ++++++++++++++++++++++++++++++++++++++++--------------
1 file changed, 56 insertions(+), 20 deletions(-)

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