Delay creation of array devices for assemble/build/create

Delay creation of array devices for assemble/build/create

We will shortly be feeding more information into the process of
creating array devices, so delay the creation.  Still open them
early if the device already exists.

This involves making sure the autof flag is in the right place
so that it can be found at creation time.

Also, Assemble, Build, and Create now always close 'mdfd'.

Signed-off-by: NeilBrown <neilb@suse.de>