SELinuxMMAC additions to perform policy versioning checks.

New construct which validates any mac_permissions.xml
policy override by checking that the version attached
to the file /data/security/current/selinux_version
jibes with the base version file /selinux_version. In
the case where those files match the policy under /data
is loaded.

Change-Id: I1e5f7cd73b432bc3a54689245388acd8a79d502e
Signed-off-by: rpcraig <rpcraig@tycho.ncsc.mil>