DNSSEC: Signing of reverse zones

Unlike primary and secondary zones, file names of reverse zones
and their configurations are different from full domain names.
So far, we ignored this difference when handling keys.

From this point on, key files are named after the configuration files
and key-gen is able to synthesize the full domain name by interpreting
the configuration. In bin/genzone, we extract the domain name from
$ORIGIN in the raw zone file.