l1 domby l2 for contains MLS constraint

As identified by Stephan Smalley, the current MLS constraint for the
contains permission of the context class should consider the current
level of a user along with the clearance level so that mls_systemlow
is no longer considered contained in mls_systemhigh.

Signed-off-by: Harry Ciao <qingtao.cao@windriver.com>

policy/mls

@@ -720,7 +720,7 @@ mlsconstrain context translate
 	(( h1 dom h2 ) or ( t1 == mlstranslate ));
 
 mlsconstrain context contains
-	( h1 dom h2 );
+	(( h1 dom h2 ) and ( l1 domby l2));
 
 #
 # MLS policy for database classes