LWIP_NETCONN_FULLDUPLEX macro

LWIP_NETCONN_FULLDUPLEX==1: Enable code that allows reading from one thread, writing from a 2nd thread and closing from a 3rd thread at the same time. ATTENTION: This is currently really alpha! Some requirements: - LWIP_NETCONN_SEM_PER_THREAD==1 is required to use one socket/netconn from multiple threads at once - sys_mbox_free() has to unblock receive tasks waiting on recvmbox/acceptmbox and prevent a task pending on this during/after deletion