MEDIUM: quic: report closing state for the MUX

MEDIUM: quic: report closing state for the MUX

Define a new API to notify the MUX from the quic-conn when the
connection is about to be closed. This happens in the following cases :
- on idle timeout
- on CONNECTION_CLOSE emission or reception

The MUX wake callback is called on these conditions. The quic-conn
QUIC_FL_NOTIFY_CLOSE is set to only report once. On the MUX side,
connection flags CO_FL_SOCK_RD_SH|CO_FL_SOCK_WR_SH are set to interrupt
future emission/reception.

This patch is the counterpart to
  "MEDIUM: mux-quic: report CO_FL_ERROR on send".
Now the quic-conn is able to report its closing, which may be translated
by the MUX into a CO_FL_ERROR on the connection for the upper layer.
This allows the MUX to properly react to the QUIC closing mechanism for
both idle-timeout and closing/draining states.