MINOR: quic: Make qc_dgrams_retransmit() return a status.

MINOR: quic: Make qc_dgrams_retransmit() return a status.

This is very helpful during retranmission when receiving ICMP port unreachable
errors after the peer has left. This is the unique case at prevent where
qc_send_hdshk_pkts() or qc_send_app_probing() may fail (when they call
qc_send_ppkts() which fails with ECONNREFUSED as errno).

Also make the callers qc_dgrams_retransmit() stop their packet process. This
is the case of quic_conn_app_io_cb() and quic_conn_io_cb().

This modifications stops definitively any packet processing when receiving
ICMP port unreachable errors.

Must be backported to 2.7.