Make local capability interface explicit. Fix #139.

Introduce a factory-, and dereference method for local capabilities. These are
capabilities that reference objects of services, which are known to be used
protection-domain internally only. To support the new Capability class methods
a protected constructor and accessor to the local object's pointer is needed
in the platform's capability base-classes. For further discussion details please
refer issue #139.

base-linux/include/base/native_types.h

@@ -108,6 +108,10 @@ namespace Genode {
 			long _tid;         /* target thread */
 			long _local_name;
 
+		protected:
+
+			Native_capability(void* ptr) : _local_name((long)ptr) { }
+
 		public:
 
 			/**
@@ -117,6 +121,8 @@ namespace Genode {
 
 			long local_name() const { return _local_name; }
 
+			void* local() const { return (void*)_local_name; }
+
 			bool valid() const { return _tid != 0; }