socket/netconn recv: FIN should only be reported once

FIN should only be reported once (as '0' for sockets, as 'ERR_CLSD' for
netconns). Before this change, ERR_CLSD was returned forever...

This is the 2nd try. First try (commit ebcae98ae6)
was buggy in that it could drop the FIN if it was read together with data
(reverted in commit ebcae98ae6).

This version fixes this by adding an apiflag and a netconn flag to keep
track of this.

src/include/lwip/api.h

@@ -64,6 +64,7 @@ extern "C" {
 #define NETCONN_MORE        0x02
 #define NETCONN_DONTBLOCK   0x04
 #define NETCONN_NOAUTORCVD  0x08 /* prevent netconn_recv_data_tcp() from updating the tcp window - must be done manually via netconn_tcp_recvd() */
+#define NETCONN_NOFIN       0x10 /* upper layer already received data, leave FIN in queue until called again */
 
 /* Flags for struct netconn.flags (u8_t) */
 /** This netconn had an error, don't block on recvmbox/acceptmbox any more */
@@ -85,7 +86,8 @@ extern "C" {
 /** Received packet info will be recorded for this netconn */
 #define NETCONN_FLAG_PKTINFO                  0x40
 #endif /* LWIP_NETBUF_RECVINFO */
-
+/** A FIN has been received but not passed to the application yet */
+#define NETCONN_FIN_RX_PENDING                0x80
 
 /* Helpers to process several netconn_types by the same code */
 #define NETCONNTYPE_GROUP(t)         ((t)&0xF0)