MINOR: quic: move IP_PKTINFO on send on a dedicated function

MINOR: quic: move IP_PKTINFO on send on a dedicated function

When using listener socket, source address for emission is explicitely
set using ancillary data for sendmsg(). This is useful to guarantee the
correct address is used when binding on a non-explicit address.

This code was implemented directly under qc_snd_buf(). However, it is
quite complex due to portability issue. For IPv4, two parallel
implementations coexist, defined under IP_PKTINFO or IP_RECVDSTADDR. For
IPv6, another option is defined under IPV6_RECVPKTINFO. Each variant
uses its distinct name which increase the code complexity.

Extract ancillary data filling in a dedicated function named
cmsg_set_saddr(). This reduces greatly the body of qc_snd_buf(). Such
functions can be replicated when other ancillary data type will be
implemented. This will notably be useful for GSO implementation.