MEDIUM: mux-quic: report CO_FL_ERROR on send

MEDIUM: mux-quic: report CO_FL_ERROR on send

Mark the connection with CO_FL_ERROR on qc_send() if the socket Tx is
closed. This flag is used by the upper layer to order a close on the
MUX. This requires to check CO_FL_ERROR in qcc_is_dead() to process to
immediate MUX free when set.

The qc_wake() callback has been completed. Most notably, it now calls
qc_send() to report a possible CO_FL_ERROR. This is useful because
qc_wake() is called by the quic-conn on imminent closing.

Note that for the moment the error flag can never be set because the
quic-conn does not report when the Tx socket is closed. This will be
implemented in a following patch.