URI-Path abbreviation in CoAP

Applications built on CoAP face a conflict between the technical need for short message sizes and the interoperability requirement of following BCP190 and thus using (relatively verbose) well-known URI paths. This document introduces a CoAP option that allows expressing well-known URI paths in as little as two bytes. Negotiating the use of this option between client and server revealed a subtle flaw in RFC7252. This document updates RFC7252 to rectify it, thus making the extension point of critical options more useful.