[kernel] Simplify mailbox code, and messages

A number of simplifications of mailboxes now that the interface is much
simpler, and synchronous.

* call and respond can now only transfer one handle at a time
* mailbox objects got rid of the message queue, and just have
  wait_queues of blocked threads, and a reply_to map.
* threads now have a message_data struct on them for use by mailboxes

src/kernel/syscalls/helpers.h

@@ -37,6 +37,12 @@ j6_status_t get_handle(j6_handle_t id, j6_cap_t caps, T *&object)
     return j6_status_ok;
 }
 
+template <typename T>
+inline j6_status_t get_handle(j6_handle_t *id, j6_cap_t caps, T *&object)
+{
+    return get_handle<T>(*id, caps, object);
+}
+
 template <>
 inline j6_status_t get_handle<obj::kobject>(j6_handle_t id, j6_cap_t caps, obj::kobject *&object)
 {