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 c1bc3be806)
(cherry picked from commit 2b0add6d1a)
(cherry picked from commit 536d6dc0c5)
(cherry picked from commit 27f777c61a)
0d126920e6