ripd: switch from sendto() to sendmsg()

ripd: switch from sendto() to sendmsg()

For unknown reasons, the IP_MULTICAST_IF sockoption doesn't
seem to work when the given socket is bound to a VRF device on
Linux. Switch from sendto() to sendmsg() so that we can specify the
outgoing interface of multicast packets using IP_PKTINFO ancillary
data. This also makes ripd more consistent with ripngd, which also
uses sendmsg() to send packets on the wire.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>