MEDIUM: conn_stream: add cs_recv() as a default rcv_buf() function

This function is generic and is able to automatically transfer data
from a conn_stream's rx buffer to the destination buffer. It does this
automatically if the mux doesn't define another rcv_buf() function.
diff --git a/src/stream_interface.c b/src/stream_interface.c
index f1b52c1..a0c73bc 100644
--- a/src/stream_interface.c
+++ b/src/stream_interface.c
@@ -1236,7 +1236,7 @@
 			break;
 		}
 
-		ret = conn->mux->rcv_buf(cs, &ic->buf, max, co_data(ic) ? CO_RFL_BUF_WET : 0);
+		ret = cs_recv(cs, &ic->buf, max, co_data(ic) ? CO_RFL_BUF_WET : 0);
 		if (cs->flags & CS_FL_RCV_MORE)
 			si->flags |= SI_FL_WAIT_ROOM;