[feladat @ 87]
Bugfix. With user level 5 or 10 and lots of zones without an owner,
like slave zones that are inserted by a supermaster, the "list all
zones" view was broken (not all zones shown, but "show page" and
"show letters" were missing as well). Function zone_count matches
zones and domains table, but users with level 5 or 10 are allowed
to see zones without an entry in the zones table as well.