RAID: Add RAID status accessibility functions

RAID:  Add RAID status accessibility functions

Similar to the way thin* accesses its kernel status, we add a method
for RAID to grab the various values in its status output without the
higher levels (LVM) having to understand how to parse the output.
Added functions include:
        - lib/activate/dev_manager.c:dev_manager_raid_status()
          Pulls the status line from the kernel

        - libdm/libdm-deptree.c:dm_get_status_raid()
          Parses status line and puts components into dm_status_raid struct

        - lib/activate/activate.c:lv_raid_dev_health()
          Accesses dm_status_raid to deliver raid dev_health string

The new structure and functions can provide a more unified way to access
status information.  ('lv_raid_percent' could switch to using these
functions, for example.)