net: implement multicast network interface API on NetBSD

[mikioh]

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

[amaitra]

I am learning go - lot of expertise on C and doing router software. Would love to contribute.

[bradfitz]

@amaitra, feel free to send changes!

[amaitra]

I have never worked on this kind of project before - so some FAQs or guidance would be appreciated.