mdadm.h: Introduce custom device policies

mdadm.h: Introduce custom device policies

The approach proposed here is to test drive policies outside
validate_geometry() separately per every drive and add determined
policies to list. The implementation reuses dev_policy we have in
mdadm.

This concept addresses following problems:
- test drives if they fit together to criteria required by metadata
  handler,
- test all drives assigned to the container even if some of them are not
  target of the request, mdmon is free to use any drive in the same
  container,
- extensibility, new policies can be added to handler easy,
- fix issues related to imsm controller domain verifying.

Add superswitch function. It is used in next patches.

Signed-off-by: Mariusz Tkaczyk <mariusz.tkaczyk@linux.intel.com>