rrset wire: Simpler DNAME decompression + maximum RDLENGTH check.

- DNAMEs are now always decompressed and then there's a check if
  compressed and decompressed sizes match, in case they shouldn't have
been decompressed in the first place.

- It is possible to create an RR with compressed DNAME in it, that would
  be actually bigger than 2**16 - 1. I've added a check for that.

- Tests for both mentioned cases pending.