Sync our copy of the timezone library with IANA release tzcode2020d.

There's no functional change at all here, but I'm curious to see
whether this change successfully shuts up Coverity's warning about
a useless strcmp(), which appeared with the previous update.

Discussion: http://mm.icann.org/pipermail/tz/2020-October/029370.html

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 2020c.
+The code in this directory is currently synced with tzcode release 2020d.
 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/zic.c

@@ -634,7 +634,8 @@ static const char *leapsec;
 static const char *tzdefault;
 
 /* -1 if the TZif output file should be slim, 0 if default, 1 if the
-   output should be fat for backward compatibility.  The default is slim.  */
+   output should be fat for backward compatibility.  ZIC_BLOAT_DEFAULT
+   determines the default.  */
 static int	bloat;
 
 static bool
@@ -787,7 +788,16 @@ main(int argc, char **argv)
 	if (optind == argc - 1 && strcmp(argv[optind], "=") == 0)
 		usage(stderr, EXIT_FAILURE);	/* usage message by request */
 	if (bloat == 0)
-		bloat = strcmp(ZIC_BLOAT_DEFAULT, "slim") == 0 ? -1 : 1;
+	{
+		static char const bloat_default[] = ZIC_BLOAT_DEFAULT;
+
+		if (strcmp(bloat_default, "slim") == 0)
+			bloat = -1;
+		else if (strcmp(bloat_default, "fat") == 0)
+			bloat = 1;
+		else
+			abort();			/* Configuration error.  */
+	}
 	if (directory == NULL)
 		directory = "data";
 	if (tzdefault == NULL)