Check that catz member zone is not a configured forward zone

Check that catz member zone is not a configured forward zone

When processing a catalog zone member zone make sure that there is no
configured pre-existing forward zone with that name.

Refactor the `dns_fwdtable_find()` function to not alter the
`DNS_R_PARTIALMATCH` result (coming from `dns_rbt_findname()`) into
`DNS_R_SUCCESS`, so that now the caller can differentiate partial
and exact matches. Patch the calling sites to expect and process
the new return value.