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

meshlink_channel_get_sendq() and meshlink_channel_get_recvq() call the
underlying UTCP connection's utcp_get_sendq() and utcp_get_recvq().
These return the amount of bytes waiting in the send and receive buffers.
In particular, a non-zero value for sendq means that sent data has not been
ACKed by the peer yet.