Re-allow specification of a new default value for an inherited column

in CREATE TABLE, but give a warning notice.  Clean up inconsistent
handling of defaults and NOT NULL flags from multiply-inherited columns.
Per pghackers discussion 28-Mar through 30-Mar.

src/test/regress/expected/create_table.out

@@ -81,6 +81,9 @@ CREATE TABLE student (
 CREATE TABLE stud_emp (
 	percent 	int4
 ) INHERITS (emp, student);
+NOTICE:  CREATE TABLE: merging multiple inherited definitions of attribute "name"
+NOTICE:  CREATE TABLE: merging multiple inherited definitions of attribute "age"
+NOTICE:  CREATE TABLE: merging multiple inherited definitions of attribute "location"
 CREATE TABLE city (
 	name		name,
 	location 	box,
@@ -132,6 +135,8 @@ CREATE TABLE c_star (
 CREATE TABLE d_star (
 	d 			float8
 ) INHERITS (b_star, c_star);
+NOTICE:  CREATE TABLE: merging multiple inherited definitions of attribute "class"
+NOTICE:  CREATE TABLE: merging multiple inherited definitions of attribute "a"
 CREATE TABLE e_star (
 	e 			int2
 ) INHERITS (c_star);

src/test/regress/expected/inherit.out

@@ -5,6 +5,8 @@ CREATE TABLE a (aa TEXT);
 CREATE TABLE b (bb TEXT) INHERITS (a);
 CREATE TABLE c (cc TEXT) INHERITS (a);
 CREATE TABLE d (dd TEXT) INHERITS (b,c,a);
+NOTICE:  CREATE TABLE: merging multiple inherited definitions of attribute "aa"
+NOTICE:  CREATE TABLE: merging multiple inherited definitions of attribute "aa"
 INSERT INTO a(aa) VALUES('aaa');
 INSERT INTO a(aa) VALUES('aaaa');
 INSERT INTO a(aa) VALUES('aaaaa');