Fixed RCODE-processing.

Extended RCODE is one 12-bit number. Until now it was not only
handled separately for old and new RCODEs, but it was also
interpreted wrong.

Unified RCODE processing - it's represented by only one number and
writing into the packet takes place at the end of query processing.

refs #300
ext-rcode: Fixed RCODE setting in error-processing
ext-rcode: Fixed error processing.
Merge branch 'master' into ext-rcode

Conflicts:
	src/knot/nameserver/process_query.c
ext-rcode: Review fixes
ext-rcode: Minor fixes to the previous fixes.
ext-rcode: Missing include
ext-rcode: Fixed wrong assert.

Some RCODEs other than NOERROR may result in DNSSEC processing,
e.g. NXDOMAIN.
Merge branch 'master' into ext-rcode
review: remove nonsensical assert
review: more code, less fairy tails
Merge branch 'ext-rcode' into 'master'

closes #300