Measure epoch of timestamp-without-time-zone from local not UTC midnight.

This patch reverts commit 191ef2b4
and thereby restores the pre-7.3 behavior of EXTRACT(EPOCH FROM
timestamp-without-tz).  Per discussion, the more recent behavior was
misguided on a couple of grounds: it makes it hard to get a
non-timezone-aware epoch value for a timestamp, and it makes this one
case dependent on the value of the timezone GUC, which is incompatible
with having timestamp_part() labeled as immutable.

The other behavior is still available (in all releases) by explicitly
casting the timestamp to timestamp with time zone before applying EXTRACT.

This will need to be called out as an incompatible change in the 9.2
release notes.  Although having mutable behavior in a function marked
immutable is clearly a bug, we're not going to back-patch such a change.

doc/src/sgml/func.sgml

@@ -6684,8 +6684,10 @@ SELECT EXTRACT(DOY FROM TIMESTAMP '2001-02-16 20:38:40');
       <term><literal>epoch</literal></term>
       <listitem>
        <para>
-        For <type>date</type> and <type>timestamp</type> values, the
+        For <type>timestamp with time zone</type> values, the
         number of seconds since 1970-01-01 00:00:00 UTC (can be negative);
+        for <type>date</type> and <type>timestamp</type> values, the
+        number of seconds since 1970-01-01 00:00:00 local time;
         for <type>interval</type> values, the total number
         of seconds in the interval
        </para>

src/backend/utils/adt/timestamp.c

@@ -4066,32 +4066,13 @@ timestamp_part(PG_FUNCTION_ARGS)
 		switch (val)
 		{
 			case DTK_EPOCH:
-				{
-					int			tz;
-					TimestampTz timestamptz;
-
-					/*
-					 * convert to timestamptz to produce consistent results
-					 */
-					if (timestamp2tm(timestamp, NULL, tm, &fsec, NULL, NULL) != 0)
-						ereport(ERROR,
-								(errcode(ERRCODE_DATETIME_VALUE_OUT_OF_RANGE),
-								 errmsg("timestamp out of range")));
-
-					tz = DetermineTimeZoneOffset(tm, session_timezone);
-
-					if (tm2timestamp(tm, fsec, &tz, &timestamptz) != 0)
-						ereport(ERROR,
-								(errcode(ERRCODE_DATETIME_VALUE_OUT_OF_RANGE),
-								 errmsg("timestamp out of range")));
-
 #ifdef HAVE_INT64_TIMESTAMP
-					result = (timestamptz - SetEpochTimestamp()) / 1000000.0;
+					result = (timestamp - SetEpochTimestamp()) / 1000000.0;
 #else
-					result = timestamptz - SetEpochTimestamp();
+					result = timestamp - SetEpochTimestamp();
 #endif
 					break;
-				}
+
 			case DTK_DOW:
 			case DTK_ISODOW:
 				if (timestamp2tm(timestamp, NULL, tm, &fsec, NULL, NULL) != 0)