is NULL then you need to preserve the 'rdata' until
                      you have finished using the structure as there may be
                      references to the associated memory.  If 'mctx' is non
                      NULL it is guarenteed that there are no references to
                      to memory addsociated with 'rdata'.

                      dns_rdata_freestruct() must be called if 'mctx' was
                      non NULL and may safely be called if 'mctx' was NULL.

Finish implementing 153 for IN specific types.
naptr_35.h some of the lengths should be 8 bits.