DHCP: protocol changes.

1. Define and add parsing code for MTU, max message size, T1, T2.
2. Add common TLVs (message size, hostname, vendor ID) to all
   packets sent by the client.
3. Don't include requested IP and server ID in renew messages,
   since the RFC says MUST NOT.
4. Don't hardcode the broadcast flag to true in DISCOVER packets,
   use what the caller passed in.
5. Make some methods static.

Bug: 19704592
Change-Id: I42a0997e468b12e19cad9b403b98fe266e6cea73