Rename zonediff_t to dns__zonediff_t and move it to lib/dns/zone_p.h, so that unit tests can be written for functions taking pointers to structures of this type as arguments. (cherry picked from commitace465a9f9) (cherry picked from commitea15c54d8a) (cherry picked from commit5712d9528f) (cherry picked from commit5f0b4eaf70)