Incremental: add --export handling.

Incremental: add --export handling.

If --export is given with --incremental, then
  MD_DEVNAME
is output which gives the name of the device (in /dev/md) that
is the array (or container) that the device would be added to.
Also
  MD_STARTED
is set to one of
  no
  unsafe
  yes
  nothing

to indicate if the array was started.  IF MD_STARTED=unsafe
then it may be appropriate to run
  mdadm -R /dev/md/$MD_DEVNAME
after a timeout to ensure newly degraded array are started.

If
  MD_FOREIGN=yes
it might be appropriate to suppress this as the array is
probably not critical.

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