sendrecv: make them two pairs of send/recv to properly deal with FTPS

FTP(S) use two connections that can be set to different recv and
send functions independently, so by introducing recv+send pairs
in the same manner we already have sockets/connections we can
work with FTPS fine.

This commit fixes the FTPS regression introduced in change d64bd82.

lib/nss.c

@@ -1339,8 +1339,8 @@ CURLcode Curl_nss_connect(struct connectdata *conn, int sockindex)
   }
 
   connssl->state = ssl_connection_complete;
-  conn->recv = nss_recv;
-  conn->send = nss_send;
+  conn->recv[sockindex] = nss_recv;
+  conn->send[sockindex] = nss_send;
 
   display_conn_info(conn, connssl->handle);