Added more safety checks to del_virtual_socket(), new zts_shutdown() implementation

src/VirtualTap.hpp

@@ -296,6 +296,11 @@ namespace ZeroTier {
 		 */
 		int Close(VirtualSocket *vs);
 
+		/*
+		 * Shuts down some aspect of a VirtualSocket
+		 */
+		int Shutdown(VirtualSocket *vs, int how);
+
 		/*
 		 * Disposes of previously-closed VirtualSockets
 		 */