Expand the allowed range of timezone offsets to +/-15:59:59 from Greenwich.

We used to only allow offsets less than +/-13 hours, then it was +/14,
then it was +/-15.  That's still not good enough though, as per today's bug
report from Patric Bechtel.  This time I actually looked through the Olson
timezone database to find the largest offsets used anywhere.  The winners
are Asia/Manila, at -15:56:00 until 1844, and America/Metlakatla, at
+15:13:42 until 1867.  So we'd better allow offsets less than +/-16 hours.

Given the history, we are way overdue to have some greppable #define
symbols controlling this, so make some ... and also remove an obsolete
comment that didn't get fixed the last time.

Back-patch to all supported branches.

src/backend/utils/adt/datetime.c

@@ -2571,9 +2571,6 @@ DecodeNumberField(int len, char *str, int fmask,
  * Return 0 if okay (and set *tzp), a DTERR code if not okay.
  *
  * NB: this must *not* ereport on failure; see commands/variable.c.
- *
- * Note: we allow timezone offsets up to 13:59.  There are places that
- * use +1300 summer time.
  */
 static int
 DecodeTimezone(char *str, int *tzp)
@@ -2618,7 +2615,8 @@ DecodeTimezone(char *str, int *tzp)
 	else
 		min = 0;
 
-	if (hr < 0 || hr > 14)
+	/* Range-check the values; see notes in utils/timestamp.h */
+	if (hr < 0 || hr > MAX_TZDISP_HOUR)
 		return DTERR_TZDISP_OVERFLOW;
 	if (min < 0 || min >= 60)
 		return DTERR_TZDISP_OVERFLOW;

src/include/utils/timestamp.h

@@ -89,6 +89,16 @@ typedef struct
 #define USECS_PER_SEC	INT64CONST(1000000)
 #endif
 
+/*
+ * We allow numeric timezone offsets up to 15:59:59 either way from Greenwich.
+ * Currently, the record holders for wackiest offsets in actual use are zones
+ * Asia/Manila, at -15:56:00 until 1844, and America/Metlakatla, at +15:13:42
+ * until 1867.  If we were to reject such values we would fail to dump and
+ * restore old timestamptz values with these zone settings.
+ */
+#define MAX_TZDISP_HOUR		15				/* maximum allowed hour part */
+#define TZDISP_LIMIT		((MAX_TZDISP_HOUR + 1) * SECS_PER_HOUR)
+
 /*
  * Macros for fmgr-callable functions.
  *