Cleanup - RDATA adjusting, rrset_deep_copy().

src/libknot/nameserver/name-server.c

src/libknot/rrset.c

src/libknot/rrset.h

src/libknot/updates/ddns.c

src/libknot/updates/xfr-in.c

src/libknot/zone/zone-contents.c

src/libknot/zone/zone-contents.h

src/libknot/zone/zone-diff.c

src/tests/libknot/rrset_tests.c