Assembe: fix bug in force_array - it wasn't forcing properly.

Since 'best' was expanded to hold replacement devices, we might
need to go up to raid_disks*2 to find devices to force.

Also fix another place when considering replacement drives would
be wrong (the 'chosen' device should never be a replacement).

Reported-by: John Yates <jyates65@gmail.com>
Signed-off-by: NeilBrown <neilb@suse.de>
Fix typos in mdadm.8.in

I found a small bug in the documentation of mdadm.  I fixed it in my
local git clone of git://neil.brown.name/mdadm  Here is the change:


Signed-off-by: NeilBrown <neilb@suse.de>
Add support for --add-spare

--add-spare is like --add, but a --re-add is never attempted.
So it is equivalent to two separate commands:

 --zero-metadata
 --add

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