MEDIUM: mux-quic: adjust transport layer error handling

MEDIUM: mux-quic: adjust transport layer error handling

Following previous patch, error notification from quic_conn has been
adjusted to rely on standard connection flags. Most notably, CO_FL_ERROR
on the connection instance when a fatal error is detected.

Check for CO_FL_ERROR is implemented by qc_send(). If set the new flag
QC_CF_ERR_CONN will be set for the MUX instance. This flag is similar to
the local error flag and will abort most of the futur processing. To
ensure stream upper layer is also notified, qc_wake_some_streams()
called by qc_process() will put the stream on error if this new flag is
set.

This should be backported up to 2.7.