153. [func] dns_rdata_tostruct() 'mxtc' is now optional. If 'mctx'
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.