Move find_zone_keys() to lib/dns/zone_p.h

Move find_zone_keys() to lib/dns/zone_p.h

Rename find_zone_keys() to dns__zone_findkeys() and move it to
lib/dns/zone_p.h, so that it can be used in unit tests.  Add a comment
describing the purpose of this function.

(cherry picked from commit d7143986b1c43884d89383ef7a2e1127e8324d33)
(cherry picked from commit fc0e99c7d7c20b1e270ab868da1a00198f39835b)
(cherry picked from commit a6be59ce082597b138fe782462a9d010bc7dd232)