pg_dump: fix dump of 4.x EXTERNAL tables with ON clauses
pg_dump: fix dump of 4.x EXTERNAL tables with ON clauses

Follow-up to 4f4e5a5c. In GPDB 4, the ON clause information is stored in
pg_exttable.location, not .command.

50wu/gpdb,xinzweb/gpdb,50wu/gpdb,adam8157/gpdb,lisakowen/gpdb,jmcatamney/gpdb,50wu/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,adam8157/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,50wu/gpdb,lisakowen/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,50wu/gpdb,greenplum-db/gpdb,lisakowen/gpdb,greenplum-db/gpdb,lisakowen/gpdb,adam8157/gpdb,xinzweb/gpdb,ashwinstar/gpdb,xinzweb/gpdb,jmcatamney/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,adam8157/gpdb,lisakowen/gpdb,ashwinstar/gpdb,lisakowen/gpdb,ashwinstar/gpdb,50wu/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,adam8157/gpdb,jmcatamney/gpdb,adam8157/gpdb,jmcatamney/gpdb,xinzweb/gpdb,xinzweb/gpdb,xinzweb/gpdb,lisakowen/gpdb,adam8157/gpdb,jmcatamney/gpdb,50wu/gpdb,lisakowen/gpdb,ashwinstar/gpdb,50wu/gpdb,xinzweb/gpdb,adam8157/gpdb,greenplum-db/gpdb,xinzweb/gpdb,ashwinstar/gpdb,greenplum-db/gpdb