dhcp_client: initialize request message for class DHCPMessage

This provides a function InitRequest to initialize a DHCP
message from client to server.
This adds a static function to generate a random transaction
id for request message.
This also introduces a few new DHCP options into class
DHCPMessage.

Bug: 25642025
TEST=compile and unittests
Change-Id: If9efdadb6c49e587cf93b5a5476136a610a15cd2
3 files changed