Fix up interactions between --assemble and --incremental
If --incremental has partly assembled an array and
--assemble is asked to assemble it, the just finds remaining
devices and makes a new array. Not good.
So:
1/ modify locking policy so that assemble can be sure that
no --incremental is running once it locks the map file
2/ Assemble() checks the map file for a duplicate and adds to
that array instead of creating a new one.