Don't allow relminmxid to go backwards during VACUUM FULL

We were allowing a table's pg_class.relminmxid value to move backwards
when heaps were swapped by VACUUM FULL or CLUSTER.  There is a
similar protection against relfrozenxid going backwards, which we
neglected to clone when the multixact stuff was rejiggered by commit
0ac5ad5134.

Backpatch to 9.3, where relminmxid was introduced.

As reported by Heikki in
http://www.postgresql.org/message-id/52401AEA.9000608@vmware.com

src/backend/commands/cluster.c

@@ -861,6 +861,12 @@ copy_heap_data(Oid OIDNewHeap, Oid OIDOldHeap, Oid OIDOldIndex, bool verbose,
 	if (TransactionIdPrecedes(FreezeXid, OldHeap->rd_rel->relfrozenxid))
 		FreezeXid = OldHeap->rd_rel->relfrozenxid;
 
+	/*
+	 * MultiXactCutoff, similarly, shouldn't go backwards either.
+	 */
+	if (MultiXactIdPrecedes(MultiXactCutoff, OldHeap->rd_rel->relminmxid))
+		MultiXactCutoff = OldHeap->rd_rel->relminmxid;
+
 	/* return selected values to caller */
 	*pFreezeXid = FreezeXid;
 	*pCutoffMulti = MultiXactCutoff;