Follow-up tweaks for issue #145

Because we use to pass a policy class to 'Native_capability_tpl'
we can pass the dst type as part of the policy instead of as
a separate template argument. This patch also adds documentation
of the POLICY interface as expected by 'Native_capability_tpl'.

base-host/include/base/native_types.h

@@ -19,8 +19,9 @@
 namespace Genode {
 
 	struct Empty_thread_id {
-		static bool valid(Empty_thread_id id) { return true; }
-		static Empty_thread_id invalid()      { return Empty_thread_id();}
+		typedef int Dst;
+		static bool valid(Dst) { return false; }
+		static Dst invalid()   { return false; }
 	};
 
 	typedef volatile int  Native_lock;