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/include/datatype/timestamp.h

@@ -105,6 +105,16 @@ typedef struct
 #define USECS_PER_MINUTE INT64CONST(60000000)
 #define USECS_PER_SEC	INT64CONST(1000000)
 
+/*
+ * 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)
+
 /*
  * DT_NOBEGIN represents timestamp -infinity; DT_NOEND represents +infinity
  */