centre does not build on FreeBSD

[vestein463]

On FreeBSD-amd64--12.2, I did not succeed in building centre[.

wb@wbk2 ~/go/src/harvey-os.org/cmd/centre]$ go build

github.com/insomniacslk/dhcp/dhcpv4/client4

/home/wb/go/pkg/mod/github.com/insomniacslk/dhcp@v0.0.0-20200814125043-2e1bf785d039/dhcpv4/client4/client.go:127:25: undefined: unix.AF_PACKET
/home/wb/go/pkg/mod/github.com/insomniacslk/dhcp@v0.0.0-20200814125043-2e1bf785d039/dhcpv4/client4/client.go:127:68: undefined: unix.ETH_P_IP
/home/wb/go/pkg/mod/github.com/insomniacslk/dhcp@v0.0.0-20200814125043-2e1bf785d039/dhcpv4/client4/client.go:135:12: undefined: unix.SockaddrLinklayer

BTW, ufs was sucessfully built in ../ufs