DHCPV4: add the DHCPV4 state machine logic part 2

This adds the second part of message handling and sending logic
for DHCPV4 state machine.
While there this also fixes a bug in DHCPMessage::InitRequest().

Bug: 25642025
TEST=compile, unittest, and run it.

Change-Id: I21892a549bd0c6c23849f9ce90c23e904ff0ef08
4 files changed