Use types with known sizes where we can, easier to serialize

Add iterator for POD and use it to implement some demarshalling.

pinos/server/link.h

@@ -69,7 +69,7 @@ struct _PinosLink {
                                 PinosPort     *port));
 
   struct {
-    unsigned int   in_ready;
+    uint32_t       in_ready;
     PinosPort     *input;
     PinosPort     *output;
     SpaList        input_link;