Remove the hack in the grammar that "optimized away" DEFAULT NULL clauses.

Instead put in a test to drop a NULL default at the last moment before
storing the catalog entry.  This changes the behavior in a couple of ways:
* Specifying DEFAULT NULL when creating an inheritance child table will
  successfully suppress inheritance of any default expression from the
  parent's column, where formerly it failed to do so.
* Specifying DEFAULT NULL for a column of a domain type will correctly
  override any default belonging to the domain; likewise for a sub-domain.
The latter change happens because by the time the clause is checked,
it won't be a simple null Const but a CoerceToDomain expression.

Personally I think this should be back-patched, but there doesn't seem to
be consensus for that on pgsql-hackers, so refraining.

src/include/parser/gramparse.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2007, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/parser/gramparse.h,v 1.38 2007/01/05 22:19:56 momjian Exp $
+ * $PostgreSQL: pgsql/src/include/parser/gramparse.h,v 1.39 2007/10/29 19:40:40 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -54,6 +54,5 @@ extern void parser_init(void);
 extern int	base_yyparse(void);
 extern List *SystemFuncName(char *name);
 extern TypeName *SystemTypeName(char *name);
-extern bool exprIsNullConstant(Node *arg);
 
 #endif   /* GRAMPARSE_H */