Add functions to get the amount of bytes in the send and receive buffers.

Add functions to get the amount of bytes in the send and receive buffers.

utcp_get_sendq() returns the amount of bytes in the send buffer, which
corresponds to the amount of bytes not yet ACKed by the peer.

utcp_get_recvq() returns the amount of bytes in the receive buffer,
which may or may not have been acked already, but have not been read by
the application.