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/backend/parser/parse_utilcmd.c

@@ -19,7 +19,7 @@
  * Portions Copyright (c) 1996-2007, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- *	$PostgreSQL: pgsql/src/backend/parser/parse_utilcmd.c,v 2.3 2007/08/27 03:36:08 tgl Exp $
+ *	$PostgreSQL: pgsql/src/backend/parser/parse_utilcmd.c,v 2.4 2007/10/29 19:40:40 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -440,7 +440,6 @@ transformColumnDefinition(ParseState *pstate, CreateStmtContext *cxt,
 							(errcode(ERRCODE_SYNTAX_ERROR),
 							 errmsg("multiple default values specified for column \"%s\" of table \"%s\"",
 								  column->colname, cxt->relation->relname)));
-				/* Note: DEFAULT NULL maps to constraint->raw_expr == NULL */
 				column->raw_default = constraint->raw_expr;
 				Assert(constraint->cooked_expr == NULL);
 				saw_default = true;