Gcov

This MR adds support to enable code coverage testing with:

```shell
./configure --enable-code-coverage
make
make check-code-coverage
```

The output can be found in $(PACKAGE_NAME)-$(PACKAGE_VERSION)-coverage directory.

You have to have lcov package installed for this to work.

DNSKEY in non-apex nodes

This changeset improves handling of DNSKEY records in non-apex nodes.

Prior to this change, non-apex DNSKEYs were not signed and invalid NSEC
records were generated for these nodes.

New Dnssec Tests

Ixfr And Ddns Fixup

This fixes:

* ixfr/add_existing
* ddns/chain

No need to be picky about this, TSIG rcode is 16bit so we might
as well use 16bit number to keep things the same.

This function releases held RCU read lock, so it is possible that
the zone is free in the meantime. To prevent accessing the freed
zone, zone is released after all processing is done and the
pointer is invalidated to prevent further access.

The problem with previous code was that the packet stripped the TSIG
section when it parsed the first IXFR packet from wire. If the code
failed though, the wire was parsed again but this time the TSIG was
already stripped on the wire so the result was garbled data.
I've thrown in some minor cleanup as well.

Zone Create No Lastnode

This makes NSEC3 nodes disappear if it is preceded by a RR with the
same owner and any other record type.

Explained in the test, BIND9 as of 9.9.4 provides wrong closest
encloser wildcard NSEC3 record.

Fix Ddns

Fixes for new tests, tests were modified a bit as well.

Improve Ddns Tests