...in accordance with RFCs 1112 and 2464.
Fixes IPv4 multicast when IP_MULTICAST_IF is specified.
Don't return ErrNoRoute when no route is needed.
Don't set Route.NextHop when no route is needed.
PiperOrigin-RevId: 236199813
Change-Id: I48ed33e1b7f760deaa37e18ad7f1b8b62819ab43