move keystores handle from the zone to the view

move keystores handle from the zone to the view

The list of keystores is owned by the single server object
(named_g_server), but dns_zone_t has a pointer into it in order to
preserve encapsulation (lib/dns won't link to bin/named for good
reasons).

However, getting the keystores from the zone uses the zone lock whereas
this is not needed (as the pointer value doesn't depends on the zone,
and is initialized only with the same named_g_server->keystores value);
also storing an extra pointer per zone is not needed; also, there was a
logic based on the zone->secure property which was not needed (as there
is only one keystore).

The keystores pointer is now accessible and lock-free at view level,
it also simplifies a bit the various zone configuration APIs (server.c,
zoneconf.c).