net: implement multicast network interface API on DragonFly BSD

[mikioh]

See https://github.com/golang/go/blob/master/src/net/interface_dragonfly.go.

[rfrangio]

Hi, I've looked a bit at this as well the netbsd/openbsd issues listed below and it really looks like it should be very simple to do using the same method used for darwin/freebsd. Most of the work should be in getting the vm's up and doing the testing. I've been looking for ways to contribute and this seems like a decent way to start. Let me know how I should proceed.

#9670
#9669