The CoAP Proxy may not know how to handle a Critical option, but if it
is safe to forward it should be able to handle this.
RFRC7252 5.4.1
Critical/elective rules apply to non-proxying endpoints. A proxy
processes options based on Unsafe/Safe-to-Forward classes as defined
in Section 5.7.
If this is an incoming proxy type request special case any
critical options that are safe to forward.
Track ongoing sessions from an enabled proxy server so unknown critical
response options can be passed back to the originating requestor.
Update coap-server to handle Proxy-Scheme for proxying.
Update tests to use critical option 25 instead of 9.
Jon Shallow
dfc678c33b