acm: Updating a policy on a running system.

acm: Updating a policy on a running system.

Allow a policy to be updated on a running system and domains to be
relabeled. The updating of a policy is happening in several steps:
relabeling the domains, testing whether the system would be in a valid
state after the relabeling (according to the policy), committing the
changes if state is determined to be valid.

I have followed Keir's suggestion of building a 2nd linked list
parallel to the domain list. That 2nd list holds security information
related to the running domains. Each entry is pointed to by its domain
structure. The list is protected by its own read/write-lock. I have
moved nearly all ACM-related code that was traversing the domain list
previously to traverse this list instead and not hold onto the domain
list lock.

Signed-off-by: Stefan Berger <stefanb@us.ibm.com>