Switch to net/ipv4 package

[ppacher]

It should be possible to use the net/ipv4 packet for the raw socket handling (currently we are using a third-party lib). Check how much efford it would be and how we could migrate to the std lib. This will likely also add support for Windows and BSD variants.
See https://godoc.org/golang.org/x/net/ipv4#RawConn
However, it seems like we will then not be able to send unicasts to specific L2 addresses as required by RFC (for example, DHCPACK is directly sent to yiaddr and the clients hwaddr without and ARP requests). However, we may still use the net/ipv4 package to support Windows and BSD variants with the limitation that those unicasts must be broadcasted. According to RFC this is allowed as well if unicasting isn't possible