DNSSEC: review fixes, mostly coding style and parameter checks

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DNSSEC: replace NSEC3 constants with inline functions

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DNSSEC: convert key name to lower case when loading the key

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DNSSEC: comments about canonical form of signed RR sets

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DNSSEC: fix error code when no key supplied for signature verification

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semantic check for RRSIG: improve info string construction

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DNSSEC: fix braces and asserts in low level signing interface

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DNSSEC: fix signer type in knot_is_valid_signature()

uint8_t * -> knot_dname_t *

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DNSSEC: proper parameter checks in zone_sign() in zone events

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