Add lib/dns/zone_p.h

Add lib/dns/zone_p.h

Add a new private header file, lib/dns/zone_p.h, which will hold type
definitions and function prototypes not meant to be exported by libdns,
but required by zone-related unit tests.

(cherry picked from commit c1bc3be80692606c13b4549d3718ad2116718d6c)
(cherry picked from commit 2b0add6d1ab2d4f173b6e37556170e07d6978272)
(cherry picked from commit 536d6dc0c59e28e288bc8a6c90ffb38558d434f3)