Cleanup the dns_catz unit API
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Change the _new, _add and _copy functions to return the new object instead of returning 'void' (or always ISC_R_SUCCESS)
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Cleanup the isc_ht_find() + isc_ht_add() usage - the code is always locked with catzs->lock (mutex), so when isc_ht_find() returns ISC_R_NOTFOUND, the isc_ht_add() must always succeed. This in turn allowed to rip out large chunk from named_server unit because the dns_catz_add_zone() would never fail even before the refactoring, it was just a bit muddled.