DNSSEC: correct signature validation in zone signing

- uses knot_dnssec_sign_verify() instead of string comparison
  and therefore OpenSSL EVP_VerifyFinal()
- previous approach could not work e.g. for DSA based ciphers

refs #4
fix: Do not commit changes to journal if there are none.
DNSSEC: use RDATA directly from validated RRSIG

- previous code constructed new RRSIG and populated it with extracted
  and processed data taken from the RRSIG which was validated
- new code uses RDATA directly from the RRSIG which is validated

refs #4
DNSSEC: ignore K prefix for key filenames

refs #4
DNSSEC: zone keys freeing cleanup
DNSSEC: disallow two zone keys with the same keytag

(at least for now)

refs #4