Generate keys with unique key tags

[Philip-NLnetLabs]

This applies to the keygen subcommand:

Having keys for a zone with key tag collisions can cause all kinds of problems. It is best to try to avoid that. The easiest is to make a list of key tags from current keys for a zone and generate a new key that has a key tag not in this list. This does not apply to the ldns-keygen mode.