Implement label compression for cat-dns

This commit implements https://github.com/notwaldorf/cat-dns/issues/6
DNS supports label compression, DNS names that appear twice or more in
a dns message can be compressed by pointing later iterations at the
first one. This commit points the answer name at the query name, since
they are always the same. As a DNS header is always 12-bytes long,
this pointer value is always the same.