BUG/MINOR: quic: handle EINTR on emission of packets without connection

BUG/MINOR: quic: handle EINTR on emission of packets without connection

Retry, Version Negotiation and Stateless Reset packets are emitted
without connection via quic_lstnr_sock_send(). If sendmsg() is
interrupted by a signal, the error is reported to the caller and the
packet is lost, while qc_snd_buf() retries the emission on EINTR.

Also retry the emission on EINTR in quic_lstnr_sock_send(). The impact
is limited, as the listener socket is non-blocking so sendmsg() is not
expected to be interrupted in practice, and a new packet will be
emitted in response to the next ones from the peer.

This may be backported along with the previous patches up to 2.8.