pgsql-server: Make ALTER TABLE ADD SERIAL work reasonably in inheritance

Log Message:
-----------
Make ALTER TABLE ADD SERIAL work reasonably in inheritance cases, too.

Modified Files:
--------------
pgsql-server/src/backend/commands:
tablecmds.c (r1.114 -> r1.115)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/commands/tablecmds.c.diff?r1=1.114&r2=1.115)