Fix forcing multicast msgs to loopback on OpenBSD.

Fix forcing multicast msgs to loopback on OpenBSD.
e.g.
$ sudo qemu -m 128 -no-fd-bootchk \
        -hda virtual.img -boot n -nographic \
        -net nic,vlan=0,model=rtl8139,macaddr=52:54:00:12:34:03 \
        -net user -tftp /usr/src/sys/arch/i386/compile/TEST -bootp pxeboot \
        -net nic,vlan=1,model=rtl8139,macaddr=52:54:00:23:03:01 \
        -net tap,vlan=1,script=no \
        -net nic,vlan=3,model=rtl8139,macaddr=52:54:00:23:03:03 \
        -net socket,vlan=3,mcast=230.0.0.1:10003
setsockopt(SOL_IP, IP_MULTICAST_LOOP): Invalid argument
qemu: -net socket,vlan=3,mcast=230.0.0.1:10003: Device 'socket' could not be initialized

Signed-off-by: Brad Smith <brad@comstyle.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
