Upgrade ALTER TABLE DROP COLUMN so that it can drop an OID column, and

remove separate implementation of ALTER TABLE SET WITHOUT OIDS in favor
of doing a regular DROP.  Also, cause CREATE TABLE to account completely
correctly for the inheritance status of the OID column.  This fixes
problems with dropping OID columns that have dependencies, as noted by
Christopher Kings-Lynne, as well as making sure that you can't drop an
OID column that was inherited from a parent.