The mapping functions between isc_result_t and dns_rcode_t could return
both isc_result_t values not defined in the header and dns_rcode_t
values not defined in the header because it blindly maps anything
withing full 12-bits defined for RCODEs to isc_result_t and back.
Refactor the dns_result_{from,to}rcode() functions to always return
valid isc_result_t and dns_rcode_t values by explicitly mapping the
values to each other and returning DNS_R_SERVFAIL (dns_rcode_servfail)
when encountering value out of the defined range.
b53d1d7069