Sync our copy of the timezone library with IANA release tzcode2018e.

The non-cosmetic changes involve teaching the "zic" tzdata compiler about
negative DST.  While I'm not currently intending that we start using
negative-DST data right away, it seems possible that somebody would try
to use our copy of zic with bleeding-edge IANA data.  So we'd better be
out in front of this change code-wise, even though it doesn't matter for
the data file we're shipping.

Discussion: https://postgr.es/m/30996.1525445902@sss.pgh.pa.us

src/timezone/README

@@ -55,7 +55,7 @@ match properly on the old version.
 Time Zone code
 ==============
 
-The code in this directory is currently synced with tzcode release 2017c.
+The code in this directory is currently synced with tzcode release 2018e.
 There are many cosmetic (and not so cosmetic) differences from the
 original tzcode library, but diffs in the upstream version should usually
 be propagated to our version.  Here are some notes about that.

src/timezone/localtime.c

@@ -60,9 +60,8 @@ static int	tzdefrules_loaded = 0;
 /*
  * The DST rules to use if TZ has no rules and we can't load TZDEFRULES.
  * Default to US rules as of 2017-05-07.
- * POSIX 1003.1 section 8.1.1 says that the default DST rules are
- * implementation dependent; for historical reasons, US rules are a
- * common default.
+ * POSIX does not specify the default DST rules;
+ * for historical reasons, US rules are a common default.
  */
 #define TZDEFRULESTRING ",M3.2.0,M11.1.0"
 
@@ -1158,10 +1157,11 @@ tzparse(const char *name, struct state *sp, bool lastditch)
 				else
 				{
 					/*
-					 * If summer time is in effect, and the transition time
-					 * was not specified as standard time, add the summer time
-					 * offset to the transition time; otherwise, add the
-					 * standard time offset to the transition time.
+					 * If daylight saving time is in effect, and the
+					 * transition time was not specified as standard time, add
+					 * the daylight saving time offset to the transition time;
+					 * otherwise, add the standard time offset to the
+					 * transition time.
 					 */
 
 					/*

src/timezone/strftime.c

@@ -203,7 +203,7 @@ _fmt(const char *format, const struct pg_tm *t, char *pt,
 					/*
 					 * Locale modifiers of C99 and later. The sequences %Ec
 					 * %EC %Ex %EX %Ey %EY %Od %oe %OH %OI %Om %OM %OS %Ou %OU
-					 * %OV %Ow %OW %Oy are supposed to provide alternate
+					 * %OV %Ow %OW %Oy are supposed to provide alternative
 					 * representations.
 					 */
 					goto label;