Fix https://github.com/zerotier/libzt/issues/244:

Memory leak if libzt events aren't consumed by user app
If arg is not enqueued by Events, then treat as if ownership has NOT
been transferred and caller of Events->enqueue is responsible for freeing

src/Events.hpp

@@ -115,8 +115,12 @@ class Events {
 
     /**
      * Enqueue an event to be sent to the user application
+     * 
+     * Returns true if arg was enqueued.
+     * If enqueued, then ownership of arg has been transferred.
+     * If NOT enqueued, then ownership of arg has NOT been transferred.
      */
-    void enqueue(unsigned int event_code, const void* arg, int len = 0);
+    bool enqueue(unsigned int event_code, const void* arg, int len = 0);
 
     /**
      * Send callback message to user application