Move zonediff_t to lib/dns/zone_p.h

Move zonediff_t to lib/dns/zone_p.h

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 commit ace465a9f924ca445eb31c925abde864a4f2adea)
(cherry picked from commit ea15c54d8a0e91ffae75f8ab2c4f244ba70665c1)
(cherry picked from commit 5712d9528f59f1383ef5b229e99172193ef57719)