1. Updated Kea guide to note that removing the hooks-libraries
configuration element does not always have the expected effect.
2. Moved some documentation from the dhcp_parsers.cc file to the .h file.
3. Expanded checking of the contents of the hooks-libraries configuration
element.
As some incompatible changes have already been made to the hooks
interface (removal of the setSkip method), it seems pointless to
support both old and new hoos-libraries syntax. These modifications
remove support for the old syntax.
