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

Sync our copy of the timezone library with IANA release tzcode2020a.

This absorbs a leap-second-related bug fix in localtime.c, and
teaches zic to handle an expiration marker in the leapseconds file.
Neither are of any interest to us (for the foreseeable future
anyway), but we need to stay more or less in sync with upstream.

Also adjust some over-eager changes in the README from commit 957338418.
I have no intention of making changes that require C99 in this code,
until such time as all the live back branches require C99.  Otherwise
back-patching will get too exciting.

For the same reason, absorb assorted whitespace and other cosmetic
changes from HEAD into the back branches; mostly this reflects use of
improved versions of pgindent.

All in all then, quite a boring update.  But I figured I'd get it
done while I was looking at this code.

Branch
------
REL9_6_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/c38561196f78a07711595b0d052608f645a75fcb

Modified Files
--------------
src/timezone/README      |   2 +-
src/timezone/localtime.c |  37 +++++++---
src/timezone/pgtz.c      |   6 +-
src/timezone/pgtz.h      |   4 +-
src/timezone/strftime.c  |  10 ++-
src/timezone/zic.c       | 181 +++++++++++++++++++++++++++++------------------
6 files changed, 153 insertions(+), 87 deletions(-)