MINOR: quic: define a function to emit a datagram on a listener socket

MINOR: quic: define a function to emit a datagram on a listener socket

Most QUIC datagrams are emitted via qc_snd_buf() which relies on a
quic_conn instance. When the connection uses the listener socket and the
listener is bound on a wildcard address, the local address retrieved on
reception is set as datagram source via ancillary data. This guarantees
that replies are emitted from the address targeted by the client, which
may otherwise drop them if its socket is connected.

However, some packets are emitted before any connection instance
exists: Retry, Version Negotiation and Stateless Reset. These are sent
via a bare sendto() on the listener socket, without source address.

Define a new function quic_lstnr_sock_send() for these packets. It
emits a datagram through a listener socket, with an extra parameter for
the local address targeted by the peer. This address is set as source
via cmsg_set_saddr() as in qc_snd_buf(), only if the listener is bound
on a wildcard address. This rule is now defined in a new function
quic_lstnr_may_set_src(), on which qc_may_use_saddr() also relies.
Apart from the source address, the emission is identical to the
existing sendto() calls: no flag is set and EINTR is not retried. As the
listener socket is non-blocking, a comment highlights that a datagram
is lost if the socket buffer is full, as there is no connection to retry
its emission later.

This patch does not introduce any functional change. It is a
prerequisite for the following fixes on Retry, Stateless Reset and
Version Negotiation emission. As such, it should be backported with
them up to 2.8.