MINOR: connection: implement cs_drain_and_close()

MINOR: connection: implement cs_drain_and_close()

We had cs_close() which forces a CS_SHR_RESET mode on the read side,
and due to this there are a few call places in the checks which
perform a manual call to conn_sock_drain() before calling cs_close().
This is absurd by principle, and it can be counter-productive in the
case of a mux where this could even cause the opposite of the desired
effect by deleting pending frames on the socket before closing.

Let's add cs_drain_and_close() which uses the CS_SHR_DRAIN mode to
prepare this.