resolve: unify memdup() code when parsing RRs

resolve: unify memdup() code when parsing RRs

Let's make dns_packet_read_public_key() more generic by renaming it to
dns_packet_read_memdup() (which more accurately describes what it
does...). Then, patch all cases where we memdup() RR data to use this
new call.

This specifically checks for zero-length objects, and handles them
gracefully. It will set zero length payload fields as a result.

Special care should be taken to ensure that any code using this call
can handle the returned allocated field to be NULL if the size is
specified as 0!