Implement ->accept_conn() callback for QUIC listener sockets.
Note that this patch also implements quic_session_accept() function
which is similar to session_accept_fd() without calling conn_complete_session()
at this time because we do not have any real QUIC mux.
We add src/quic_sock.c QUIC specific socket management functions as callbacks
for the control layer: ->accept_conn, ->default_iocb and ->rx_listening.
accept_conn() will have to be defined. The default I/O handler only recvfrom()
the datagrams received. Furthermore, ->rx_listening callback always returns 1 at
this time but should returns 0 when reloading the processus.