Implement "WITH / WITHOID OIDS" clause for CREATE TABLE AS. This is

Implement "WITH / WITHOID OIDS" clause for CREATE TABLE AS. This is
intended to allow application authors to insulate themselves from
changes to the default value of 'default_with_oids' in future releases
of PostgreSQL.

This patch also fixes a bug in the earlier implementation of the
'default_with_oids' GUC variable: code in gram.y should not examine
the value of GUC variables directly due to synchronization issues.