Add failfast support.

Add failfast support.

Allow per-device "failfast" flag to be set when creating an
array or adding devices to an array.

When re-adding a device which had the failfast flag, it can be removed
using --nofailfast.

failfast status is printed in --detail and --examine output.

Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>