Add optional "validator" function to languages that can validate the

function body (and other properties) as a function in the language
is created.  This generalizes ad hoc code that already existed for
the built-in languages.

The validation now happens after the pg_proc tuple of the new function
is created, so it is possible to define recursive SQL functions.

Add some regression test cases that cover bogus function definition
attempts.

src/include/nodes/parsenodes.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2001, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $Id: parsenodes.h,v 1.178 2002/05/17 18:32:52 petere Exp $
+ * $Id: parsenodes.h,v 1.179 2002/05/22 17:21:01 petere Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -990,6 +990,7 @@ typedef struct CreatePLangStmt
 	NodeTag		type;
 	char	   *plname;			/* PL name */
 	List	   *plhandler;		/* PL call handler function (qual. name) */
+	List	   *plvalidator;	/* optional validator function (qual. name) */
 	char	   *plcompiler;		/* lancompiler text */
 	bool		pltrusted;		/* PL is trusted */
 } CreatePLangStmt;