Make isc_result tables smaller

Make isc_result tables smaller

The isc_result_t enum was to sparse when each library code would skip to
next << 16 as a base.  Remove the huge holes in the isc_result_t enum to
make the isc_result tables more compact.

This change required a rewrite how we map dns_rcode_t to isc_result_t
and back, so we don't ever return neither isc_result_t value nor
dns_rcode_t out of defined range.

(cherry picked from commit a8e6c3b8f7b919075cae6db1d9835318c0350fc3)