Add -fail support to --incremental

This can be used for hot-unplug.  When a device has been remove,
udev can call
   mdadm --incremental --fail sda

and mdadm will find the array holding sda and remove sda from
the array.

Based on code from  Doug Ledford <dledford@redhat.com>

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