Fix a bug in configure_catz_zone()

Fix a bug in configure_catz_zone()

When dns_catz_zone_add() returns ISC_R_EXISTS and there is no
'default-primaries' or 'default-masters', the ISC_R_EXISTS result
code doesn't get reset to ISC_R_SUCCESS, and the function returns
ISC_R_EXISTS instead of ISC_R_SUCCESS. Which means that the zone
is successfully added, but the caller assumes that the function has
failed.

Reset 'result' to ISC_R_SUCCESS when dns_catz_zone_add() returns
ISC_R_EXISTS (it's not an error condition).

Refactor the code go call dns_catz_zone_add() when all other error
conditions are already checked.