This patch absorbs a few unreleased fixes in the IANA code.
It corresponds to commit 2d8b944c1cec0808ac4f7a9ee1a463c28f9cd00a
in https://github.com/eggert/tz.  Non-cosmetic changes include:

TZDEFRULESTRING is updated to match current US DST practice,
rather than what it was over ten years ago.  This only matters
for interpretation of POSIX-style zone names (e.g., "EST5EDT"),
and only if the timezone database doesn't include either an exact
match for the zone name or a "posixrules" entry.  The latter
should not be true in any current Postgres installation, but
this could possibly matter when using --with-system-tzdata.

Get rid of a nonportable use of "++var" on a bool var.
This is part of a larger fix that eliminates some vestigial
support for consecutive leap seconds, and adds checks to
the "zic" compiler that the data files do not specify that.

Remove a couple of ancient compatibility hacks.  The IANA
crew think these are obsolete, and I tend to agree.  But
perhaps our buildfarm will think different.

Back-patch to all supported branches, in line with our policy
that all branches should be using current IANA code.  Before v10,
this includes application of current pgindent rules, to avoid
whitespace problems in future back-patches.

Discussion: https://postgr.es/m/E1dsWhf-0000pT-F9@gemulon.postgresql.org