RFC conformity and label compression for cat-dns

This change addresses https://github.com/notwaldorf/cat-dns/issues/4, https://github.com/notwaldorf/cat-dns/issues/5 and https://github.com/notwaldorf/cat-dns/issues/6 . 

With these changes, cat-dns now includes the original question, works with EDNS0 options (try dig +bufsize=4096 www.example.com @localhost) and makes the answer name a compressed pointer to the query name.