DHCPMessage: support DHCP option 'parameter request list'

This adds the support for DHCP option 'parameter request
list'. This allows the client to request specified
configuration parameters from the DHCP server.

There are some other minor changes:
Replace 'GetData' with 'GetConstData' in DeviceInfo unittest.
Add a missing 'return false' in DHCPMessage.
Add DHCP message length checking in DHCPMessage
::Serialize().

Bug: 25642025
TEST=compile and unittest
Change-Id: I6e9d756aa2925ab2390b0a1096ae3d663b241aba
4 files changed