MINOR: quic: remove sendto() usage variant

MINOR: quic: remove sendto() usage variant

qc_snd_buf() is a wrapper around emission syscalls. Given QUIC
configuration, a different variant is used. When using connection
socket, send() is the only used. For listener sockets, sendmsg() and
sendto() are possible. The first one is used only if local address has
been retrieved prior. This allows to fix it on sending to guarantee the
source address selection. Finally, sendto() is used for systems which do
not support local address retrieval.

All of these variants render the code too complex. As such, this patch
simplifies this by removing sendto() alternative. Now, sendmsg() is
always used for listener sockets. Source address is then specified only
if supported by the system.

This patch should not exhibit functional behavior changes. It will be
useful when implementing GSO as the code is now simpler.