New built-in client class for incomplete unpacking
Current Kea accepts packets which have a not fatal error during unpacking. I believe it was added by @tmark: in such case the SkipRemainingOptionsError exception is thrown and processing continue.
I'd like to put such packets in a new built-in class so a "not option[xxx].exist" can't be mislead: it will be enough to add "add not member("')".
This allows too to classify such packets in the DROP class so by configuration accept or drop them.