change issecuredomain() functions to bool

change issecuredomain() functions to bool

dns_keytable_issecuredomain() and dns_view_issecuredomain()
previously returned a result code to inform the caller of
unexpected database failures when looking up names in the
keytable and/or NTA table. such failures are not actually
possible. both functions now return a simple bool.

also, dns_view_issecuredomain() now returns false if
view->enablevalidation is false, so the caller no longer
has to check for that.