Implement nested block comments in the backend and in psql.

Include updates for the comment.sql regression test.
Implement SET SESSION CHARACTERISTICS and SET DefaultXactIsoLevel.
Implement SET SESSION CHARACTERISTICS TRANSACTION COMMIT
 and SET AutoCommit in the parser only.
 Need to add code to actually do something.
Implement WITHOUT TIME ZONE type qualifier.
Define SCHEMA keyword, along with stubbed-out grammar.
Implement "[IN|INOUT|OUT] [varname] type" function arguments
 in parser only; INOUT and OUT throws an elog(ERROR).
Add PATH as a type-specific token, since PATH is in SQL99
 to support schema resource search and resolution.

src/include/nodes/parsenodes.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2000, PostgreSQL, Inc
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $Id: parsenodes.h,v 1.108 2000/06/12 03:41:03 momjian Exp $
+ * $Id: parsenodes.h,v 1.109 2000/07/14 15:43:51 thomas Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -688,6 +688,16 @@ typedef struct ExplainStmt
 	bool		verbose;		/* print plan info */
 } ExplainStmt;
 
+/* ----------------------
+ *		Set Session Statement
+ * ----------------------
+ */
+typedef struct SetSessionStmt
+{
+	NodeTag		type;
+	List	   *args;
+} SetSessionStmt;
+
 /* ----------------------
  * Set Statement
  * ----------------------