sockets: change closing: netconn is freed when socket is closed, not before

This is necessary to implement fullduplex sockets that are closed asynchronously:
the netconn in the socket must not be freed before all threads have given up
using it.

We now call the first part of 'netconn_delete()' (moved to 'netconn_prepare_delete()')
from lwip_close() and only actually end up calling 'netconn_free()' from
'free_socket()', which might be called later if LWIP_NETCONN_FULLDUPLEX is enabled.

Signed-off-by: goldsimon <goldsimon@gmx.de>

src/include/lwip/api.h

@@ -73,6 +73,10 @@ extern "C" {
 #define NETCONN_FLAG_NON_BLOCKING             0x02
 /** Was the last connect action a non-blocking one? */
 #define NETCONN_FLAG_IN_NONBLOCKING_CONNECT   0x04
+#if LWIP_NETCONN_FULLDUPLEX
+  /** The mbox of this netconn is being deallocated, don't use it anymore */
+#define NETCONN_FLAG_MBOXINVALID              0x08
+#endif /* LWIP_NETCONN_FULLDUPLEX */
 /** If a nonblocking write has been rejected before, poll_tcp needs to
     check if the netconn is writable again */
 #define NETCONN_FLAG_CHECK_WRITESPACE         0x10
@@ -302,6 +306,7 @@ struct netvector {
 #define netconn_new_with_callback(t, c) netconn_new_with_proto_and_callback(t, 0, c)
 struct netconn *netconn_new_with_proto_and_callback(enum netconn_type t, u8_t proto,
                                              netconn_callback callback);
+err_t   netconn_prepare_delete(struct netconn *conn);
 err_t   netconn_delete(struct netconn *conn);
 /** Get the type of a netconn (as enum netconn_type). */
 #define netconn_type(conn) (conn->type)