trustrouter: permit trust-router-COI and error handling
Improve the error handling of the trust router integration. Set
Error-Cause to indicate a proxy routing failure if we fail to get a
realm via trustrouter. Include the error back from the trust router
in Reply-Message.
Set trust-router-coi in the trust router code rather than depending on
unlang to do so. Permit a previously specified trust-router-coi in
the request to override the community in the configuration. Unlang is
responsible for performing any authorization checks regarding
community.