Add support for "--re-add missing"

Add support for "--re-add missing"

If the device name "missing" is given for --re-add, then mdadm will
attempt to find any device which should be a member of the array but
currently isn't and will --re-add it to the array.
This can be useful if a device disappeared due to a cabling problem,
and was then re-connected.
The appropriate sequence would be
  mdadm /dev/mdX --fail detached
  mdadm /dev/mdX --remove detached
  mdadm /dev/mdX --re-add missing

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