libknot/dnssec now contains interface for:
- key loading
- low level DNSSEC signing
- NSEC3 hash computation
- SIG(0)
- generating NSEC/NSEC3 records for zones
- generating RRSIG for zones

- KNOT_NODE_FLAGS_OLD and KNOT_NODE_FLAGS_NEW is not used
- KNOT_NODE_FLAGS_EMPTY is used only in dname.c now, but is already
  removed in dname refactoring branch

(knot_zone_t -> knot_zone_contents_t)

- no functional changes
- outdated descriptions were removed/updated
- reduced number of necessary iterations over the zone nodes

- 'dnssec-keydir' is set for all zones
- 'dnssec-enable' is set for all zones, can be overridden for individual zone

- connections are generated when NSEC3 nodes are created
- explicitly request node connecting when the zone is loaded or after incoming transfer

- always use OpenSSL EVP
- do not touch output parameters if there is an error
- do not abort when there is an allocation error
- prepare for additional hash algorithms
- remove debug outputs

refs #2413

Change-Id: I0b0a572eb5cc11787e332404226b0b079ed85f88

refs #2413

Change-Id: I3d97bc4ea2f0986900c503bd10bc5f5ceb2585a6

The public key is specified as DNS RR. Before this patch, only first
line of the input file was read even if it contained comment, white
spaces or just a part of the RR.

refs #2413

Change-Id: I4cf3374121fbb676d89a52fa4ba741e497b04564

refs #2413

Change-Id: I44e0e6b9058781d58c54ee8521e294836ee4e183

refs #2413

Change-Id: I071d0a4560ded8051352ee6d30513a8c10b51a02

Allows cleaning up of the digest context to create multiple signatures.

refs #2413

Change-Id: Idcc23b99269d266d2ed1cbb6abda0bc73ec07338

refs #2413

Change-Id: I8c40b8bdf39bff7cf4e5d6ae41cb2612b634b932