DHCP: glue code.

1. Add a method to make a DhcpResults object from a DHCP packet.
2. Add a method to fetch the client MAC from the packet. This is
   needed to check that the message is for us (lots of DHCP
   messages are broadcast).
3. Add a length argument to the method that parses DHCP packets,
   so the caller can use the same MTU-sized array all the time
   instead of having to pass in a new array for every packet.

Bug: 19704592
Change-Id: I58223f5ec90fb5c762bc2934649e02f9122018b2