Add IS UNKNOWN, IS NOT UNKNOWN boolean tests, fix the existing boolean

tests to return the correct results per SQL9x when given NULL inputs.
Reimplement these tests as well as IS [NOT] NULL to have their own
expression node types, instead of depending on special functions.
From Joe Conway, with a little help from Tom Lane.

src/backend/parser/keywords.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/parser/keywords.c,v 1.92 2001/05/08 21:06:43 petere Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/parser/keywords.c,v 1.93 2001/06/19 22:39:11 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -265,6 +265,7 @@ static ScanKeyword ScanKeywords[] = {
 	{"type", TYPE_P},
 	{"union", UNION},
 	{"unique", UNIQUE},
+	{"unknown", UNKNOWN},
 	{"unlisten", UNLISTEN},
 	{"until", UNTIL},
 	{"update", UPDATE},