MINOR: quic: use qc_send_hdshk_pkts() in handshake IO cb

MINOR: quic: use qc_send_hdshk_pkts() in handshake IO cb

quic_conn_io_cb() manually implements emission by using lower level
functions qc_prep_pkts() and qc_send_ppkts(). Replace this by using the
higher level function qc_send_hdshk_pkts() which notably handle buffer
allocation and purging.

This allows to clean up send API by flagging qc_prep_pkts() and
qc_send_ppkts() as static. They are now used in a single location inside
qc_send_hdshk_pkts().