Move timeofday() implementation out of nabstime.c.

nabstime.c is about to be removed, but timeofday() isn't related to
the rest of the functionality therein, and some find it useful. Move
to timestamp.c.

Discussion:
    https://postgr.es/m/20181009192237.34wjp3nmw7oynmmr@alap3.anarazel.de
    https://postgr.es/m/20180928223240.kgwc4czzzekrpsid%40alap3.anarazel.de

src/backend/utils/adt/nabstime.c

@@ -1539,32 +1539,3 @@ bogus:
 					"tinterval", i_string)));
 	*i_start = *i_end = INVALID_ABSTIME;	/* keep compiler quiet */
 }
-
-
-/*****************************************************************************
- *
- *****************************************************************************/
-
-/*
- * timeofday -
- *	   returns the current time as a text. similar to timenow() but returns
- *	   seconds with more precision (up to microsecs). (I need this to compare
- *	   the Wisconsin benchmark with Illustra whose TimeNow() shows current
- *	   time with precision up to microsecs.)			  - ay 3/95
- */
-Datum
-timeofday(PG_FUNCTION_ARGS)
-{
-	struct timeval tp;
-	char		templ[128];
-	char		buf[128];
-	pg_time_t	tt;
-
-	gettimeofday(&tp, NULL);
-	tt = (pg_time_t) tp.tv_sec;
-	pg_strftime(templ, sizeof(templ), "%a %b %d %H:%M:%S.%%06d %Y %Z",
-				pg_localtime(&tt, session_timezone));
-	snprintf(buf, sizeof(buf), templ, tp.tv_usec);
-
-	PG_RETURN_TEXT_P(cstring_to_text(buf));
-}

src/backend/utils/adt/timestamp.c

@@ -1609,6 +1609,26 @@ GetSQLLocalTimestamp(int32 typmod)
 	return ts;
 }
 
+/*
+ * timeofday(*) -- returns the current time as a text.
+ */
+Datum
+timeofday(PG_FUNCTION_ARGS)
+{
+	struct timeval tp;
+	char		templ[128];
+	char		buf[128];
+	pg_time_t	tt;
+
+	gettimeofday(&tp, NULL);
+	tt = (pg_time_t) tp.tv_sec;
+	pg_strftime(templ, sizeof(templ), "%a %b %d %H:%M:%S.%%06d %Y %Z",
+				pg_localtime(&tt, session_timezone));
+	snprintf(buf, sizeof(buf), templ, tp.tv_usec);
+
+	PG_RETURN_TEXT_P(cstring_to_text(buf));
+}
+
 /*
  * TimestampDifference -- convert the difference between two timestamps
  *		into integer seconds and microseconds