Smack: Integrate Smack with Audit

Setup the new Audit hooks for Smack. SELinux Audit rule fields are recycled
to avoid `auditd' userspace modifications. Currently only equality testing
is supported on labels acting as a subject (AUDIT_SUBJ_USER) or as an object
(AUDIT_OBJ_USER).

Signed-off-by: Ahmed S. Darwish <darwish.07@gmail.com>
Acked-by: Casey Schaufler <casey@schaufler-ca.com>