Add policy framework.

Add policy framework.

Policy can be stated as lines in mdadm.conf like:

POLICY  type=disk path=pci-0000:00:1f.2-* action=ignore domain=onboard

This defines two distinct policies which apply to any disk (but not
partition) device reached through the pci device 0000:00:1f.2.
The policies are "action=ignore" which means certain actions will
ignore the device, and "domain=onboard" which means all such devices
as treated as being united under the name 'onboard'.

This patch just adds data structures and code to read and
manipulate them.  Future patches will actually use them.

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