Allow to boot from a pool within which replacing is in progress. Before the change it wasn't possible and the following error was printed:
Allow to boot from a pool within which replacing is in progress.
Before the change it wasn't possible and the following error was printed:

	ZFS: can only boot from disk, mirror or raidz vdevs

Now if the original vdev (the one we are replacing) is still present we will
read from it, but if it is not present we won't read from the new vdev, as it
might not have enough valid data yet.

MFC after:	2 weeks

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase