Implement named-checkconf -k (check keys)

Implement named-checkconf -k (check keys)

With named-checkconf -k you can check your configuration including
checking the dnssec-policy keys against the configured keystores. If
there is a mismatch in the key files versus the policy, named-checkconf
will fail. This is useful for running before migrating to dnssec-policy.

For logging purposes, introduce a function that writes the identifying
information about a policy key into a string.

Allow a dnssec key to be initialized outside the keymgr code.

Add 'log_errors' to 'cfg_kasp_fromconfig' to avoid duplicate error
logs.