Change NSEC3 and NSEC3PARAM struct fields to use isc_region_t

Change NSEC3 and NSEC3PARAM struct fields to use isc_region_t

Replace the separate pointer+length field pairs in dns_rdata_nsec3_t
(salt/salt_length, next/next_length, typebits/len) and
dns_rdata_nsec3param_t (salt/salt_length) with isc_region_t.  This
makes the structs self-describing and eliminates a class of
length-mismatch bugs.

The dns_zone_setnsec3param() signature is updated to take
isc_region_t *salt instead of separate saltlen and salt arguments.

Function signatures for dns_nsec3_addnsec3, dns_db_getnsec3parameters,
and related internal functions still use separate pointer+length pairs
and should be updated in a follow-up.