nsupdate generates correct UPDATE packets, better I/O

src/libknot/packet/packet.c

src/libknot/packet/packet.h

src/utils/common/resolv.c

src/utils/common/resolv.h

src/utils/nsupdate/nsupdate_exec.c

src/utils/nsupdate/nsupdate_params.c

src/utils/nsupdate/nsupdate_params.h