Allow --incremental to add spares to an array.

Allow --incremental to add spares to an array.

Commit 3a6ec29ad56 stopped us from adding apparently-working devices
to an active array with --incremental as there is a good chance that they
are actually old/failed devices.

Unfortunately it also stopped spares from being added to an active
array, which is wrong.  This patch refines the test to be more
careful.

Reported-by: <fibreraid@gmail.com>
Analysed-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>