Always send the DHCP client identifier.

Currently we send it only in request packets, but not in discover
packets. This might confuse servers because they might think that
the discover and the request come from different clients.

Also reorder the options in the request packet to match the order
used by the legacy DHCP client.

While I'm at it, fix the generation code for inform and decline
packets, which we do not use.

Bug: 19704592
Bug: 20335221
Change-Id: I1d45306e76dbd5da9cc4611e6df84a9f67346b2c