Implement access control

Move send and dispatch functions to the implementation. This makes it
possible to place an access check before sending and dispatching.
Add module-access that allows to bind and notify on globals owned by
the client.

spa/include/spa/defs.h

@@ -67,6 +67,8 @@ typedef enum {
   SPA_RESULT_ASYNC_BUSY                = -30,
   SPA_RESULT_INVALID_OBJECT_ID         = -31,
   SPA_RESULT_NO_MEMORY                 = -32,
+  SPA_RESULT_NO_PERMISSION             = -33,
+  SPA_RESULT_SKIPPED                   = -34,
 } SpaResult;
 
 #define SPA_ASYNC_MASK                  (3 << 30)