DHCPV4: make and send dhcp packet

This adds two functions.
One is MakeRawPacket(), extracting payload from DHCPMessage
object and filling the IP and UDP headers.

Another is SendRawPacket(), sending the dhcp packet through
raw socket.

Bug: 25642025
TEST=compile and unittest

Change-Id: Ib9efca5d96b768757d26e70119564b98e819bb34
4 files changed