DHCPMessaget: support DHCP option 'message'

This adds the support for DHCP option 'message'.
This allows both the client and server to indicate the
reason of failure using a string.

While there, this also fixes a few typos.
Bug: 25642025
TEST=compile and unittest

Change-Id: Ie678923c14ccbaea8e4801418c919c07108cfded
3 files changed