Introduce a new DNSSEC tool, dnssec-ksr, for creating signed key
response (SKR) files, given one or more key signing requests (KSRs).
For now it is just a dummy tool, but the future purpose of this utility
is to pregenerate ZSKs and signed RRsets for DNSKEY, CDNSKEY, and CDS
for a given period that a KSK is to be offline.