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

src/picoTCP.hpp

@@ -192,6 +192,11 @@ namespace ZeroTier
 		 */
 		int pico_Close(VirtualSocket *vs);
 
+		/*
+		 *  Shuts down some aspect of a VirtualSocket - Called from VirtualTap
+		 */
+		int pico_Shutdown(VirtualSocket *vs, int how);
+
 		/*
 		 * Converts a pico_err to its most closely-related errno, and sets errno
 		 */