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.

src/timezone/pgtz.h

@@ -49,10 +49,16 @@ struct state
 	pg_time_t	ats[TZ_MAX_TIMES];
 	unsigned char types[TZ_MAX_TIMES];
 	struct ttinfo ttis[TZ_MAX_TYPES];
-	char		chars[BIGGEST(BIGGEST(TZ_MAX_CHARS + 1, 3 /* sizeof gmt */ ),
+	char		chars[BIGGEST(BIGGEST(TZ_MAX_CHARS + 1, 4 /* sizeof gmt */ ),
 							  (2 * (TZ_STRLEN_MAX + 1)))];
 	struct lsinfo lsis[TZ_MAX_LEAPS];
-	int			defaulttype;	/* for early times or if no transitions */
+
+	/*
+	 * The time type to use for early times or if no transitions. It is always
+	 * zero for recent tzdb releases. It might be nonzero for data from tzdb
+	 * 2018e or earlier.
+	 */
+	int			defaulttype;
 };