Introduce a new XPRT method, start(). The init() method will now only
initialize whatever is needed for the XPRT to run, but any action the XPRT
has to do before being ready, such as handshakes, will be done in the new
start() method. That way, we will be sure the full stack of xprt will be
initialized before attempting to do anything.
The init() call is also moved to conn_prepare(). There's no longer any reason
to wait for the ctrl to be ready, any action will be deferred until start(),
anyway. This means conn_xprt_init() is no longer needed.
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.
