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
diff --git a/dhcp_options.h b/dhcp_options.h
index 1a9f498..8257585 100644
--- a/dhcp_options.h
+++ b/dhcp_options.h
@@ -28,6 +28,7 @@
 const uint8_t kDHCPOptionMessageType = 53;
 const uint8_t kDHCPOptionServerIdentifier = 54;
 const uint8_t kDHCPOptionParameterRequestList = 55;
+const uint8_t kDHCPOptionMessage = 56;
 const uint8_t kDHCPOptionRenewalTime = 58;
 const uint8_t kDHCPOptionRebindingTime = 59;
 const uint8_t kDHCPOptionClientIdentifier = 61;