[3033] Addressed review comments.

Removed configuration parameter allow-client-update.
It was deemed to really just be the inverse of override-client-update.
Renamed convenience method isDhcpDdns to ddnsEnabled.
Made enum to string converion functions case insenstive.
Updated copyright dates and other cosmetic changes.
[3033] Addressed addition review comments.

Changed IP addresses used in tests in dhcp_parsers_unittest.cc
to comply test values recommened by RFCs.