Define a new quic-initial action named "reject". Contrary to dgram-drop,
the client is notified of the rejection by a CONNECTION_CLOSE with
CONNECTION_REFUSED error code.
To be able to emit the necessary CONNECTION_CLOSE frame, quic_conn is
instantiated, contrary to dgram-drop action. quic_set_connection_close()
is called immediatly after qc_new_conn() which prevents the handshake
startup.
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