pg_upgrade:  remove pg_multixact files left by initdb
pg_upgrade:  remove pg_multixact files left by initdb

This fixes a bug that caused vacuum to fail when the '0000' files left
by initdb were accessed as part of vacuum's cleanup of old pg_multixact
files.

Backpatch through 9.3

xinzweb/gpdb,zeroae/postgres-xl,50wu/gpdb,ashwinstar/gpdb,Postgres-XL/Postgres-XL,jmcatamney/gpdb,xinzweb/gpdb,pavanvd/postgres-xl,lisakowen/gpdb,50wu/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,Postgres-XL/Postgres-XL,lisakowen/gpdb,techdragon/Postgres-XL,ashwinstar/gpdb,Postgres-XL/Postgres-XL,ovr/postgres-xl,adam8157/gpdb,lisakowen/gpdb,zeroae/postgres-xl,50wu/gpdb,Postgres-XL/Postgres-XL,jmcatamney/gpdb,xinzweb/gpdb,oberstet/postgres-xl,yazun/postgres-xl,zeroae/postgres-xl,greenplum-db/gpdb,lisakowen/gpdb,greenplum-db/gpdb,xinzweb/gpdb,jmcatamney/gpdb,adam8157/gpdb,oberstet/postgres-xl,xinzweb/gpdb,jmcatamney/gpdb,adam8157/gpdb,50wu/gpdb,ovr/postgres-xl,yazun/postgres-xl,adam8157/gpdb,jmcatamney/gpdb,lisakowen/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,adam8157/gpdb,yazun/postgres-xl,ovr/postgres-xl,lisakowen/gpdb,50wu/gpdb,greenplum-db/gpdb,techdragon/Postgres-XL,yazun/postgres-xl,jmcatamney/gpdb,lisakowen/gpdb,greenplum-db/gpdb,50wu/gpdb,ovr/postgres-xl,yazun/postgres-xl,greenplum-db/gpdb,pavanvd/postgres-xl,techdragon/Postgres-XL,techdragon/Postgres-XL,xinzweb/gpdb,greenplum-db/gpdb,oberstet/postgres-xl,adam8157/gpdb,zeroae/postgres-xl,zeroae/postgres-xl,techdragon/Postgres-XL,pavanvd/postgres-xl,greenplum-db/gpdb,xinzweb/gpdb,ovr/postgres-xl,ashwinstar/gpdb,pavanvd/postgres-xl,50wu/gpdb,xinzweb/gpdb,Postgres-XL/Postgres-XL,adam8157/gpdb,adam8157/gpdb,ashwinstar/gpdb,oberstet/postgres-xl,pavanvd/postgres-xl,lisakowen/gpdb,50wu/gpdb,ashwinstar/gpdb,oberstet/postgres-xl