Merge from vendor branch TNF:

Merge from vendor branch TNF:
ZKT is a tool to manage keys and signatures for DNSSEC-zones.
The Zone Key Tool consist of two commands:
* dnssec-zkt to create and list dnssec zone keys and
* dnssec-signer to sign a zone and manage the lifetime of the zone signing keys
Both commands are simple wrapper commands around the dnssec-keygen(8) and
dnssec-signzone(8) commands provided by BIND.