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

Sync our copy of the timezone library with IANA release tzcode2018f.

About half of this is purely cosmetic changes to reduce the diff between
our code and theirs, like inserting "const" markers where they have them.

The other half is tracking actual code changes in zic.c and localtime.c.
I don't think any of these represent near-term compatibility hazards, but
it seems best to stay up to date.

I also fixed longstanding bugs in our code for producing the
known_abbrevs.txt list, which by chance hadn't been exposed before,
but which resulted in some garbage output after applying the upstream
changes in zic.c. Notably, because upstream removed their old phony
transitions at the Big Bang, it's now necessary to cope with TZif files
containing no DST transition times at all.

Branch
------
REL_10_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/11359db354b2f62e1da433da482657ff0a5ac180

Modified Files
--------------
src/timezone/README | 2 +-
src/timezone/localtime.c | 126 ++++++++++++------
src/timezone/pgtz.h | 10 +-
src/timezone/private.h | 3 +
src/timezone/strftime.c | 10 +-
src/timezone/tzfile.h | 7 +-
src/timezone/zic.c | 330 ++++++++++++++++++++++++++---------------------
7 files changed, 291 insertions(+), 197 deletions(-)