normalize syntax checks between named and libisccfg

normalize syntax checks between named and libisccfg

there were some duplicated syntax checks in named_zone_configure()
that are no longer needed, now that we perform those same checks
using isccfg_check_zoneconf().

there were also some syntax checks that were *only* in
named_zone_configure(), which have now been moved to
isccfg_check_zoneconf(). test cases for them have been
added to the checkconf system test.