label: Introduce LabelOps to do pre/post labelling operations

By default, label_ops is initialized with a NULL pointer which translates
to noop labelling operations. In mac_selinux_init() and the new mac_smack_init(),
we initialize label_ops with a MAC specific LabelOps pointer.

We also introduce mac_init() to initialize any configured MACs and replace all
usages of mac_selinux_init() with mac_init().

src/sysusers/sysusers.c

@@ -2178,7 +2178,7 @@ static int run(int argc, char *argv[]) {
 
         umask(0022);
 
-        r = mac_selinux_init();
+        r = mac_init();
         if (r < 0)
                 return r;