Add unit test to ensure the right NSEC3PARAM event is scheduled in
'dns_zone_setnsec3param()'. To avoid scheduling and managing actual
tasks, split up the 'dns_zone_setnsec3param()' function in two parts:
1. 'dns__zone_lookup_nsec3param()' that will check if the requested
NSEC3 parameters already exist, and if a new salt needs to be
generated.
2. The actual scheduling of the new NSEC3PARAM event (if needed).
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.
Rename update_sigs() to dns__zone_updatesigs() and move it to
lib/dns/zone_p.h, so that it can be unit tested. Add a comment
describing the purpose of this function.
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.
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.
