Add const qualifier to tzn returned by timestamp2tm()

The tzn value might come from tm->tm_zone, which libc declares as
const, so it's prudent that the upper layers know about this as well.

src/backend/utils/adt/datetime.c

@@ -4337,7 +4337,7 @@ pg_timezone_names(PG_FUNCTION_ARGS)
 	int			tzoff;
 	struct pg_tm tm;
 	fsec_t		fsec;
-	char	   *tzn;
+	const char *tzn;
 	Interval   *resInterval;
 	struct pg_tm itm;
 

src/backend/utils/adt/formatting.c

@@ -459,7 +459,7 @@ typedef struct TmToChar
 {
 	struct pg_tm tm;			/* classic 'tm' struct */
 	fsec_t		fsec;			/* fractional seconds */
-	char	   *tzn;			/* timezone */
+	const char *tzn;			/* timezone */
 } TmToChar;
 
 #define tmtcTm(_X)	(&(_X)->tm)

src/backend/utils/adt/timestamp.c

@@ -494,7 +494,7 @@ timestamptz_out(PG_FUNCTION_ARGS)
 	struct pg_tm tt,
 			   *tm = &tt;
 	fsec_t		fsec;
-	char	   *tzn;
+	const char *tzn;
 	char		buf[MAXDATELEN + 1];
 
 	if (TIMESTAMP_NOT_FINITE(dt))
@@ -1415,7 +1415,7 @@ timestamptz_to_str(TimestampTz t)
 	struct pg_tm tt,
 			   *tm = &tt;
 	fsec_t		fsec;
-	char	   *tzn;
+	const char *tzn;
 
 	if (TIMESTAMP_NOT_FINITE(t))
 		EncodeSpecialTimestamp(t, buf);
@@ -1466,7 +1466,7 @@ dt2time(Timestamp jd, int *hour, int *min, int *sec, fsec_t *fsec)
  * timezone) will be used.
  */
 int
-timestamp2tm(Timestamp dt, int *tzp, struct pg_tm * tm, fsec_t *fsec, char **tzn, pg_tz *attimezone)
+timestamp2tm(Timestamp dt, int *tzp, struct pg_tm * tm, fsec_t *fsec, const char **tzn, pg_tz *attimezone)
 {
 	Timestamp	date;
 	Timestamp	time;
@@ -1602,7 +1602,7 @@ recalc_t:
 		tm->tm_zone = tx->tm_zone;
 		*tzp = -tm->tm_gmtoff;
 		if (tzn != NULL)
-			*tzn = (char *) tm->tm_zone;
+			*tzn = tm->tm_zone;
 	}
 	else
 	{

src/backend/utils/adt/xml.c

@@ -2013,7 +2013,7 @@ map_sql_value_to_xml_value(Datum value, Oid type, bool xml_escape_strings)
 					struct pg_tm tm;
 					int			tz;
 					fsec_t		fsec;
-					char	   *tzn = NULL;
+					const char *tzn = NULL;
 					char		buf[MAXDATELEN + 1];
 
 					timestamp = DatumGetTimestamp(value);

src/include/utils/timestamp.h

@@ -222,7 +222,7 @@ extern const char *timestamptz_to_str(TimestampTz t);
 
 extern int	tm2timestamp(struct pg_tm * tm, fsec_t fsec, int *tzp, Timestamp *dt);
 extern int timestamp2tm(Timestamp dt, int *tzp, struct pg_tm * tm,
-			 fsec_t *fsec, char **tzn, pg_tz *attimezone);
+			 fsec_t *fsec, const char **tzn, pg_tz *attimezone);
 extern void dt2time(Timestamp dt, int *hour, int *min, int *sec, fsec_t *fsec);
 
 extern int	interval2tm(Interval span, struct pg_tm * tm, fsec_t *fsec);

src/interfaces/ecpg/pgtypeslib/timestamp.c

@@ -119,7 +119,7 @@ SetEpochTimestamp(void)
  *	local time zone. If out of this range, leave as GMT. - tgl 97/05/27
  */
 static int
-timestamp2tm(timestamp dt, int *tzp, struct tm * tm, fsec_t *fsec, char **tzn)
+timestamp2tm(timestamp dt, int *tzp, struct tm * tm, fsec_t *fsec, const char **tzn)
 {
 #ifdef HAVE_INT64_TIMESTAMP
 	int64		dDate,
@@ -224,7 +224,7 @@ recalc_t:
 
 			*tzp = -tm->tm_gmtoff;		/* tm_gmtoff is Sun/DEC-ism */
 			if (tzn != NULL)
-				*tzn = (char *) tm->tm_zone;
+				*tzn = tm->tm_zone;
 #elif defined(HAVE_INT_TIMEZONE)
 			*tzp = (tm->tm_isdst > 0) ? TIMEZONE_GLOBAL - SECS_PER_HOUR : TIMEZONE_GLOBAL;
 			if (tzn != NULL)