Separation message / request-response layer in options processing

[chrysn]

The prescribed behavior for processing critical options (to wit, reject when it happens in a NON but 4.02 in CON) has repeatedly tripped up people.

As any client already needs to be prepared to receive the 4.02 anyway (after all, the server could be behind a reverse proxy that sent CON or used reliable transport, and then wouldn't know the precise cause and respond NON 4.02), I suggest that expected behavior be changed to reject with 4.02 independently of the transport, and just to allow a server to completely reject requests that are not understood for compatibility reasons.