dhcp client: add checksum computing function for class DHCPMessages

When using raw sockets for communicating with the DHCP
server, we need to fill the IP and UDP headers. Adding
checksum function allows us to fill the checksum field
in headers.

Bug: 25642025
TEST=compile and unittests
Change-Id: Icab3433ccb2b1fffbf1fb4ce60362d1b94588235
6 files changed